Vendor import of libc++ trunk r351319 (just before the release_80vendor/libc++/libc++-trunk-r351319

branch point):
https://llvm.org/svn/llvm-project/libcxx/trunk@351319

86 files changed, 4527 insertions, 1717 deletions

diff --git a/include/CMakeLists.txt b/include/CMakeLists.txt
index d9def18d725c..73f7cfc4d8e3 100644
--- a/include/CMakeLists.txt
+++ b/include/CMakeLists.txt
@@ -25,6 +25,7 @@ set(files
   any
   array
   atomic
+  bit
   bitset
   cassert
   ccomplex
@@ -68,7 +69,6 @@ set(files
   experimental/chrono
   experimental/coroutine
   experimental/deque
-  experimental/dynarray
   experimental/filesystem
   experimental/forward_list
   experimental/functional
diff --git a/include/__bit_reference b/include/__bit_reference
index 3e4a21d261ff..c208af2b4d76 100644
--- a/include/__bit_reference
+++ b/include/__bit_reference
@@ -12,6 +12,7 @@
 #define _LIBCPP___BIT_REFERENCE
 
 #include <__config>
+#include <bit>
 #include <algorithm>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -254,18 +255,18 @@ __count_bool_true(__bit_iterator<_Cp, _IsConst> __first, typename _Cp::size_type
         __storage_type __clz_f = static_cast<__storage_type>(__bits_per_word - __first.__ctz_);
         __storage_type __dn = _VSTD::min(__clz_f, __n);
         __storage_type __m = (~__storage_type(0) << __first.__ctz_) & (~__storage_type(0) >> (__clz_f - __dn));
-        __r = _VSTD::__pop_count(*__first.__seg_ & __m);
+        __r = _VSTD::__popcount(*__first.__seg_ & __m);
         __n -= __dn;
         ++__first.__seg_;
     }
     // do middle whole words
     for (; __n >= __bits_per_word; ++__first.__seg_, __n -= __bits_per_word)
-        __r += _VSTD::__pop_count(*__first.__seg_);
+        __r += _VSTD::__popcount(*__first.__seg_);
     // do last partial word
     if (__n > 0)
     {
         __storage_type __m = ~__storage_type(0) >> (__bits_per_word - __n);
-        __r += _VSTD::__pop_count(*__first.__seg_ & __m);
+        __r += _VSTD::__popcount(*__first.__seg_ & __m);
     }
     return __r;
 }
@@ -285,18 +286,18 @@ __count_bool_false(__bit_iterator<_Cp, _IsConst> __first, typename _Cp::size_typ
         __storage_type __clz_f = static_cast<__storage_type>(__bits_per_word - __first.__ctz_);
         __storage_type __dn = _VSTD::min(__clz_f, __n);
         __storage_type __m = (~__storage_type(0) << __first.__ctz_) & (~__storage_type(0) >> (__clz_f - __dn));
-        __r = _VSTD::__pop_count(~*__first.__seg_ & __m);
+        __r = _VSTD::__popcount(~*__first.__seg_ & __m);
         __n -= __dn;
         ++__first.__seg_;
     }
     // do middle whole words
     for (; __n >= __bits_per_word; ++__first.__seg_, __n -= __bits_per_word)
-        __r += _VSTD::__pop_count(~*__first.__seg_);
+        __r += _VSTD::__popcount(~*__first.__seg_);
     // do last partial word
     if (__n > 0)
     {
         __storage_type __m = ~__storage_type(0) >> (__bits_per_word - __n);
-        __r += _VSTD::__pop_count(~*__first.__seg_ & __m);
+        __r += _VSTD::__popcount(~*__first.__seg_ & __m);
     }
     return __r;
 }
diff --git a/include/__config b/include/__config
index 639d06c9f5d7..f0bc3483facd 100644
--- a/include/__config
+++ b/include/__config
@@ -33,14 +33,10 @@
 #  define _GNUC_VER_NEW 0
 #endif
 
-#define _LIBCPP_VERSION 7000
+#define _LIBCPP_VERSION 8000
 
 #ifndef _LIBCPP_ABI_VERSION
-#  ifdef __Fuchsia__
-#    define _LIBCPP_ABI_VERSION 2
-#  else
-#    define _LIBCPP_ABI_VERSION 1
-#  endif
+#  define _LIBCPP_ABI_VERSION 1
 #endif
 
 #ifndef _LIBCPP_STD_VER
@@ -99,6 +95,8 @@
 // Use the smallest possible integer type to represent the index of the variant.
 // Previously libc++ used "unsigned int" exclusivly.
 #  define _LIBCPP_ABI_VARIANT_INDEX_TYPE_OPTIMIZATION
+// Unstable attempt to provide a more optimized std::function
+#  define _LIBCPP_ABI_OPTIMIZED_FUNCTION
 #elif _LIBCPP_ABI_VERSION == 1
 #  if !defined(_LIBCPP_OBJECT_FORMAT_COFF)
 // Enable compiling copies of now inline methods into the dylib to support
@@ -123,7 +121,9 @@
 #define _LIBCPP_CONCAT1(_LIBCPP_X,_LIBCPP_Y) _LIBCPP_X##_LIBCPP_Y
 #define _LIBCPP_CONCAT(_LIBCPP_X,_LIBCPP_Y) _LIBCPP_CONCAT1(_LIBCPP_X,_LIBCPP_Y)
 
-#define _LIBCPP_NAMESPACE _LIBCPP_CONCAT(__,_LIBCPP_ABI_VERSION)
+#ifndef _LIBCPP_ABI_NAMESPACE
+# define _LIBCPP_ABI_NAMESPACE _LIBCPP_CONCAT(__,_LIBCPP_ABI_VERSION)
+#endif
 
 #if __cplusplus < 201103L
 #define _LIBCPP_CXX03_LANG
@@ -328,6 +328,43 @@
 #  define _LIBCPP_NO_CFI
 #endif
 
+#if __ISO_C_VISIBLE >= 2011 || __cplusplus >= 201103L
+#  if defined(__FreeBSD__)
+#    define _LIBCPP_HAS_QUICK_EXIT
+#    define _LIBCPP_HAS_C11_FEATURES
+#  elif defined(__Fuchsia__)
+#    define _LIBCPP_HAS_QUICK_EXIT
+#    define _LIBCPP_HAS_TIMESPEC_GET
+#    define _LIBCPP_HAS_C11_FEATURES
+#  elif defined(__linux__)
+#    if !defined(_LIBCPP_HAS_MUSL_LIBC)
+#      if _LIBCPP_GLIBC_PREREQ(2, 15) || defined(__BIONIC__)
+#        define _LIBCPP_HAS_QUICK_EXIT
+#      endif
+#      if _LIBCPP_GLIBC_PREREQ(2, 17)
+#        define _LIBCPP_HAS_C11_FEATURES
+#        define _LIBCPP_HAS_TIMESPEC_GET
+#      endif
+#    else // defined(_LIBCPP_HAS_MUSL_LIBC)
+#      define _LIBCPP_HAS_QUICK_EXIT
+#      define _LIBCPP_HAS_TIMESPEC_GET
+#      define _LIBCPP_HAS_C11_FEATURES
+#    endif
+#  endif // __linux__
+#endif
+
+#ifndef _LIBCPP_CXX03_LANG
+# define _LIBCPP_ALIGNOF(_Tp) alignof(_Tp)
+#elif defined(_LIBCPP_COMPILER_CLANG)
+# define _LIBCPP_ALIGNOF(_Tp) _Alignof(_Tp)
+#else
+// This definition is potentially buggy, but it's only taken with GCC in C++03,
+// which we barely support anyway. See llvm.org/PR39713
+# define _LIBCPP_ALIGNOF(_Tp) __alignof(_Tp)
+#endif
+
+#define _LIBCPP_PREFERRED_ALIGNOF(_Tp) __alignof(_Tp)
+
 #if defined(_LIBCPP_COMPILER_CLANG)
 
 // _LIBCPP_ALTERNATE_STRING_LAYOUT is an old name for
@@ -342,7 +379,7 @@
 #  define _ALIGNAS_TYPE(x) alignas(x)
 #  define _ALIGNAS(x) alignas(x)
 #else
-#  define _ALIGNAS_TYPE(x) __attribute__((__aligned__(__alignof(x))))
+#  define _ALIGNAS_TYPE(x) __attribute__((__aligned__(_LIBCPP_ALIGNOF(x))))
 #  define _ALIGNAS(x) __attribute__((__aligned__(x)))
 #endif
 
@@ -430,28 +467,6 @@ typedef __char32_t char32_t;
 #define _LIBCPP_HAS_NO_VARIABLE_TEMPLATES
 #endif
 
-#if __ISO_C_VISIBLE >= 2011 || __cplusplus >= 201103L
-#  if defined(__FreeBSD__)
-#    define _LIBCPP_HAS_QUICK_EXIT
-#    define _LIBCPP_HAS_C11_FEATURES
-#  elif defined(__Fuchsia__)
-#    define _LIBCPP_HAS_QUICK_EXIT
-#    define _LIBCPP_HAS_C11_FEATURES
-#  elif defined(__linux__)
-#    if !defined(_LIBCPP_HAS_MUSL_LIBC)
-#      if _LIBCPP_GLIBC_PREREQ(2, 15) || defined(__BIONIC__)
-#        define _LIBCPP_HAS_QUICK_EXIT
-#      endif
-#      if _LIBCPP_GLIBC_PREREQ(2, 17)
-#        define _LIBCPP_HAS_C11_FEATURES
-#      endif
-#    else // defined(_LIBCPP_HAS_MUSL_LIBC)
-#      define _LIBCPP_HAS_QUICK_EXIT
-#      define _LIBCPP_HAS_C11_FEATURES
-#    endif
-#  endif // __linux__
-#endif
-
 #if !(__has_feature(cxx_noexcept))
 #define _LIBCPP_HAS_NO_NOEXCEPT
 #endif
@@ -464,16 +479,6 @@ typedef __char32_t char32_t;
 #define _LIBCPP_IS_LITERAL(T) __is_literal(T)
 #endif
 
-// Inline namespaces are available in Clang regardless of C++ dialect.
-#define _LIBCPP_BEGIN_NAMESPACE_STD namespace std {inline namespace _LIBCPP_NAMESPACE {
-#define _LIBCPP_END_NAMESPACE_STD  } }
-#define _VSTD std::_LIBCPP_NAMESPACE
-
-namespace std {
-  inline namespace _LIBCPP_NAMESPACE {
-  }
-}
-
 #if !defined(_LIBCPP_HAS_NO_ASAN) && !__has_feature(address_sanitizer)
 #define _LIBCPP_HAS_NO_ASAN
 #endif
@@ -493,10 +498,15 @@ namespace std {
 
 #define _LIBCPP_ALWAYS_INLINE __attribute__ ((__always_inline__))
 
+// No apple compilers support ""d and ""y at this time.
+#if _LIBCPP_CLANG_VER < 800 || defined(__apple_build_version__)
+#define	_LIBCPP_HAS_NO_CXX20_CHRONO_LITERALS
+#endif
+
 #elif defined(_LIBCPP_COMPILER_GCC)
 
 #define _ALIGNAS(x) __attribute__((__aligned__(x)))
-#define _ALIGNAS_TYPE(x) __attribute__((__aligned__(__alignof(x))))
+#define _ALIGNAS_TYPE(x) __attribute__((__aligned__(_LIBCPP_ALIGNOF(x))))
 
 #define _LIBCPP_NORETURN __attribute__((noreturn))
 
@@ -563,15 +573,6 @@ namespace std {
 
 #endif  // __GXX_EXPERIMENTAL_CXX0X__
 
-#define _LIBCPP_BEGIN_NAMESPACE_STD namespace std { inline namespace _LIBCPP_NAMESPACE {
-#define _LIBCPP_END_NAMESPACE_STD  } }
-#define _VSTD std::_LIBCPP_NAMESPACE
-
-namespace std {
-  inline namespace _LIBCPP_NAMESPACE {
-  }
-}
-
 #if !defined(_LIBCPP_HAS_NO_ASAN) && !defined(__SANITIZE_ADDRESS__)
 #define _LIBCPP_HAS_NO_ASAN
 #endif
@@ -607,13 +608,6 @@ namespace std {
 #define _ALIGNAS_TYPE(x) alignas(x)
 #define _LIBCPP_HAS_NO_VARIADICS
 
-#define _LIBCPP_BEGIN_NAMESPACE_STD namespace std {
-#define _LIBCPP_END_NAMESPACE_STD  }
-#define _VSTD std
-
-namespace std {
-}
-
 #define _LIBCPP_WEAK
 
 #define _LIBCPP_HAS_NO_ASAN
@@ -625,7 +619,7 @@ namespace std {
 #elif defined(_LIBCPP_COMPILER_IBM)
 
 #define _ALIGNAS(x) __attribute__((__aligned__(x)))
-#define _ALIGNAS_TYPE(x) __attribute__((__aligned__(__alignof(x))))
+#define _ALIGNAS_TYPE(x) __attribute__((__aligned__(_LIBCPP_ALIGNOF(x))))
 #define _ATTRIBUTE(x) __attribute__((x))
 #define _LIBCPP_NORETURN __attribute__((noreturn))
 
@@ -641,15 +635,6 @@ namespace std {
 #define __MULTILOCALE_API
 #endif
 
-#define _LIBCPP_BEGIN_NAMESPACE_STD namespace std {inline namespace _LIBCPP_NAMESPACE {
-#define _LIBCPP_END_NAMESPACE_STD  } }
-#define _VSTD std::_LIBCPP_NAMESPACE
-
-namespace std {
-  inline namespace _LIBCPP_NAMESPACE {
-  }
-}
-
 #define _LIBCPP_HAS_NO_ASAN
 
 #define _LIBCPP_ALWAYS_INLINE __attribute__ ((__always_inline__))
@@ -658,20 +643,6 @@ namespace std {
 
 #endif // _LIBCPP_COMPILER_[CLANG|GCC|MSVC|IBM]
 
-#if _LIBCPP_STD_VER >= 17
-#define _LIBCPP_BEGIN_NAMESPACE_FILESYSTEM \
-  _LIBCPP_BEGIN_NAMESPACE_STD inline namespace __fs { namespace filesystem {
-#else
-#define _LIBCPP_BEGIN_NAMESPACE_FILESYSTEM \
-  _LIBCPP_BEGIN_NAMESPACE_STD namespace __fs { namespace filesystem {
-#endif
-
-#define _LIBCPP_END_NAMESPACE_FILESYSTEM \
-  _LIBCPP_END_NAMESPACE_STD } }
-
-#define _VSTD_FS _VSTD::__fs::filesystem
-
-
 #if defined(_LIBCPP_OBJECT_FORMAT_COFF)
 
 #ifdef _DLL
@@ -685,33 +656,29 @@ namespace std {
 #  define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
 #  define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
 #  define _LIBCPP_OVERRIDABLE_FUNC_VIS
+#  define _LIBCPP_EXPORTED_FROM_ABI
 #elif defined(_LIBCPP_BUILDING_LIBRARY)
 #  define _LIBCPP_DLL_VIS __declspec(dllexport)
 #  define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
 #  define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS _LIBCPP_DLL_VIS
 #  define _LIBCPP_OVERRIDABLE_FUNC_VIS _LIBCPP_DLL_VIS
+#  define _LIBCPP_EXPORTED_FROM_ABI __declspec(dllexport)
 #else
 #  define _LIBCPP_DLL_VIS __declspec(dllimport)
 #  define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS _LIBCPP_DLL_VIS
 #  define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
 #  define _LIBCPP_OVERRIDABLE_FUNC_VIS
+#  define _LIBCPP_EXPORTED_FROM_ABI __declspec(dllimport)
 #endif
 
 #define _LIBCPP_TYPE_VIS            _LIBCPP_DLL_VIS
 #define _LIBCPP_FUNC_VIS            _LIBCPP_DLL_VIS
-#define _LIBCPP_EXTERN_VIS          _LIBCPP_DLL_VIS
 #define _LIBCPP_EXCEPTION_ABI       _LIBCPP_DLL_VIS
 #define _LIBCPP_HIDDEN
 #define _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
 #define _LIBCPP_TEMPLATE_VIS
 #define _LIBCPP_ENUM_VIS
 
-#if defined(_LIBCPP_COMPILER_MSVC)
-#  define _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY __forceinline
-#else
-#  define _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY __attribute__ ((__always_inline__))
-#endif
-
 #endif // defined(_LIBCPP_OBJECT_FORMAT_COFF)
 
 #ifndef _LIBCPP_HIDDEN
@@ -759,8 +726,12 @@ namespace std {
 #  endif
 #endif
 
-#ifndef _LIBCPP_EXTERN_VIS
-#define _LIBCPP_EXTERN_VIS
+#ifndef _LIBCPP_EXPORTED_FROM_ABI
+#  if !defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
+#    define _LIBCPP_EXPORTED_FROM_ABI __attribute__((__visibility__("default")))
+#  else
+#    define _LIBCPP_EXPORTED_FROM_ABI
+#  endif
 #endif
 
 #ifndef _LIBCPP_OVERRIDABLE_FUNC_VIS
@@ -801,21 +772,63 @@ namespace std {
 #  define _LIBCPP_INTERNAL_LINKAGE _LIBCPP_ALWAYS_INLINE
 #endif
 
-#ifndef _LIBCPP_HIDE_FROM_ABI
-#  define _LIBCPP_HIDE_FROM_ABI _LIBCPP_HIDDEN _LIBCPP_INTERNAL_LINKAGE
+#if __has_attribute(exclude_from_explicit_instantiation)
+#  define _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION __attribute__ ((__exclude_from_explicit_instantiation__))
+#else
+   // Try to approximate the effect of exclude_from_explicit_instantiation
+   // (which is that entities are not assumed to be provided by explicit
+   // template instantitations in the dylib) by always inlining those entities.
+#  define _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION _LIBCPP_ALWAYS_INLINE
 #endif
 
-// Just so we can migrate to _LIBCPP_HIDE_FROM_ABI gradually.
-#define _LIBCPP_INLINE_VISIBILITY _LIBCPP_HIDE_FROM_ABI
+#ifndef _LIBCPP_HIDE_FROM_ABI_PER_TU
+#  ifndef _LIBCPP_HIDE_FROM_ABI_PER_TU_BY_DEFAULT
+#    define _LIBCPP_HIDE_FROM_ABI_PER_TU 0
+#  else
+#    define _LIBCPP_HIDE_FROM_ABI_PER_TU 1
+#  endif
+#endif
 
-#ifndef _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
-#  if !defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
-#    define _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY __attribute__((__visibility__("default"), __always_inline__))
+#ifndef _LIBCPP_HIDE_FROM_ABI
+#  if _LIBCPP_HIDE_FROM_ABI_PER_TU
+#    define _LIBCPP_HIDE_FROM_ABI _LIBCPP_HIDDEN _LIBCPP_INTERNAL_LINKAGE
 #  else
-#    define _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY __attribute__((__always_inline__))
+#    define _LIBCPP_HIDE_FROM_ABI _LIBCPP_HIDDEN _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION
 #  endif
 #endif
 
+#ifdef _LIBCPP_BUILDING_LIBRARY
+#  if _LIBCPP_ABI_VERSION > 1
+#    define _LIBCPP_HIDE_FROM_ABI_AFTER_V1 _LIBCPP_HIDE_FROM_ABI
+#  else
+#    define _LIBCPP_HIDE_FROM_ABI_AFTER_V1
+#  endif
+#else
+#  define _LIBCPP_HIDE_FROM_ABI_AFTER_V1 _LIBCPP_HIDE_FROM_ABI
+#endif
+
+// Just so we can migrate to the new macros gradually.
+#define _LIBCPP_INLINE_VISIBILITY _LIBCPP_HIDE_FROM_ABI
+
+// Inline namespaces are available in Clang/GCC/MSVC regardless of C++ dialect.
+#define _LIBCPP_BEGIN_NAMESPACE_STD namespace std { inline namespace _LIBCPP_ABI_NAMESPACE {
+#define _LIBCPP_END_NAMESPACE_STD  } }
+#define _VSTD std::_LIBCPP_ABI_NAMESPACE
+_LIBCPP_BEGIN_NAMESPACE_STD _LIBCPP_END_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 17
+#define _LIBCPP_BEGIN_NAMESPACE_FILESYSTEM \
+  _LIBCPP_BEGIN_NAMESPACE_STD inline namespace __fs { namespace filesystem {
+#else
+#define _LIBCPP_BEGIN_NAMESPACE_FILESYSTEM \
+  _LIBCPP_BEGIN_NAMESPACE_STD namespace __fs { namespace filesystem {
+#endif
+
+#define _LIBCPP_END_NAMESPACE_FILESYSTEM \
+  _LIBCPP_END_NAMESPACE_STD } }
+
+#define _VSTD_FS _VSTD::__fs::filesystem
+
 #ifndef _LIBCPP_PREFERRED_OVERLOAD
 #  if __has_attribute(__enable_if__)
 #    define _LIBCPP_PREFERRED_OVERLOAD __attribute__ ((__enable_if__(true, "")))
@@ -976,7 +989,14 @@ template <unsigned> struct __static_assert_check {};
 // If we are getting operator new from the MSVC CRT, then allocation overloads
 // for align_val_t were added in 19.12, aka VS 2017 version 15.3.
 #if defined(_LIBCPP_MSVCRT) && defined(_MSC_VER) && _MSC_VER < 1912
-#define _LIBCPP_HAS_NO_ALIGNED_ALLOCATION
+#  define _LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION
+#elif defined(_LIBCPP_ABI_MICROSOFT) && !defined(_LIBCPP_NO_VCRUNTIME)
+#  define _LIBCPP_DEFER_NEW_TO_VCRUNTIME
+#  if !defined(__cpp_aligned_new)
+     // We're defering to Microsoft's STL to provide aligned new et al. We don't
+     // have it unless the language feature test macro is defined.
+#    define _LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION
+#  endif
 #endif
 
 #if defined(__APPLE__)
@@ -984,13 +1004,13 @@ template <unsigned> struct __static_assert_check {};
       defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__)
 #    define __MAC_OS_X_VERSION_MIN_REQUIRED __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__
 #  endif
-#  if defined(__MAC_OS_X_VERSION_MIN_REQUIRED)
-#    if __MAC_OS_X_VERSION_MIN_REQUIRED < 1060
-#      define _LIBCPP_HAS_NO_ALIGNED_ALLOCATION
-#    endif
-#  endif
 #endif // defined(__APPLE__)
 
+#if !defined(_LIBCPP_HAS_NO_ALIGNED_ALLOCATION) && \
+    (defined(_LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION) || \
+    (!defined(__cpp_aligned_new) || __cpp_aligned_new < 201606))
+#  define _LIBCPP_HAS_NO_ALIGNED_ALLOCATION
+#endif
 
 #if defined(__APPLE__) || defined(__FreeBSD__)
 #define _LIBCPP_HAS_DEFAULTRUNELOCALE
@@ -1000,18 +1020,46 @@ template <unsigned> struct __static_assert_check {};
 #define _LIBCPP_WCTYPE_IS_MASK
 #endif
 
-#if _LIBCPP_STD_VER > 11
-#  define _LIBCPP_DEPRECATED [[deprecated]]
+#if _LIBCPP_STD_VER <= 17 || !defined(__cpp_char8_t)
+#define _LIBCPP_NO_HAS_CHAR8_T
+#endif
+
+// Deprecation macros.
+// Deprecations warnings are only enabled when _LIBCPP_ENABLE_DEPRECATION_WARNINGS is defined.
+#if defined(_LIBCPP_ENABLE_DEPRECATION_WARNINGS)
+#  if __has_attribute(deprecated)
+#    define _LIBCPP_DEPRECATED __attribute__ ((deprecated))
+#  elif _LIBCPP_STD_VER > 11
+#    define _LIBCPP_DEPRECATED [[deprecated]]
+#  else
+#    define _LIBCPP_DEPRECATED
+#  endif
 #else
 #  define _LIBCPP_DEPRECATED
 #endif
 
+#if !defined(_LIBCPP_CXX03_LANG)
+#  define _LIBCPP_DEPRECATED_IN_CXX11 _LIBCPP_DEPRECATED
+#else
+#  define _LIBCPP_DEPRECATED_IN_CXX11
+#endif
+
+#if _LIBCPP_STD_VER >= 14
+#  define _LIBCPP_DEPRECATED_IN_CXX14 _LIBCPP_DEPRECATED
+#else
+#  define _LIBCPP_DEPRECATED_IN_CXX14
+#endif
+
+#if _LIBCPP_STD_VER >= 17
+#  define _LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_DEPRECATED
+#else
+#  define _LIBCPP_DEPRECATED_IN_CXX17
+#endif
+
 #if _LIBCPP_STD_VER <= 11
 #  define _LIBCPP_EXPLICIT_AFTER_CXX11
-#  define _LIBCPP_DEPRECATED_AFTER_CXX11
 #else
 #  define _LIBCPP_EXPLICIT_AFTER_CXX11 explicit
-#  define _LIBCPP_DEPRECATED_AFTER_CXX11 [[deprecated]]
 #endif
 
 #if _LIBCPP_STD_VER > 11 && !defined(_LIBCPP_HAS_NO_CXX14_CONSTEXPR)
@@ -1032,8 +1080,30 @@ template <unsigned> struct __static_assert_check {};
 #  define _LIBCPP_CONSTEXPR_AFTER_CXX17
 #endif
 
-#if __has_cpp_attribute(nodiscard) && _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17)
-#  define _LIBCPP_NODISCARD_AFTER_CXX17 [[nodiscard]]
+// The _LIBCPP_NODISCARD_ATTRIBUTE should only be used to define other
+// NODISCARD macros to the correct attribute.
+#if __has_cpp_attribute(nodiscard) || defined(_LIBCPP_COMPILER_MSVC)
+#  define _LIBCPP_NODISCARD_ATTRIBUTE [[nodiscard]]
+#elif defined(_LIBCPP_COMPILER_CLANG) && !defined(_LIBCPP_CXX03_LANG)
+#  define _LIBCPP_NODISCARD_ATTRIBUTE [[clang::warn_unused_result]]
+#else
+// We can't use GCC's [[gnu::warn_unused_result]] and
+// __attribute__((warn_unused_result)), because GCC does not silence them via
+// (void) cast.
+#  define _LIBCPP_NODISCARD_ATTRIBUTE
+#endif
+
+// _LIBCPP_NODISCARD_EXT may be used to apply [[nodiscard]] to entities not
+// specified as such as an extension.
+#if defined(_LIBCPP_ENABLE_NODISCARD) && !defined(_LIBCPP_DISABLE_NODISCARD_EXT)
+#  define _LIBCPP_NODISCARD_EXT _LIBCPP_NODISCARD_ATTRIBUTE
+#else
+#  define _LIBCPP_NODISCARD_EXT
+#endif
+
+#if !defined(_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17) && \
+    (_LIBCPP_STD_VER > 17 || defined(_LIBCPP_ENABLE_NODISCARD))
+#  define _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_NODISCARD_ATTRIBUTE
 #else
 #  define _LIBCPP_NODISCARD_AFTER_CXX17
 #endif
@@ -1090,6 +1160,7 @@ _LIBCPP_FUNC_VIS extern "C" void __sanitizer_annotate_contiguous_container(
       defined(__Fuchsia__) || \
       defined(__NetBSD__) || \
       defined(__linux__) || \
+      defined(__GNU__) || \
       defined(__APPLE__) || \
       defined(__CloudABI__) || \
       defined(__sun__) || \
@@ -1195,8 +1266,12 @@ _LIBCPP_FUNC_VIS extern "C" void __sanitizer_annotate_contiguous_container(
 #  define _LIBCPP_DIAGNOSE_ERROR(...)
 #endif
 
-#if __has_attribute(fallthough) || _GNUC_VER >= 700
 // Use a function like macro to imply that it must be followed by a semicolon
+#if __cplusplus > 201402L && __has_cpp_attribute(fallthrough)
+#  define _LIBCPP_FALLTHROUGH() [[fallthrough]]
+#elif __has_cpp_attribute(clang::fallthrough)
+#  define _LIBCPP_FALLTHROUGH() [[clang::fallthrough]]
+#elif __has_attribute(fallthough) || _GNUC_VER >= 700
 #  define _LIBCPP_FALLTHROUGH() __attribute__((__fallthrough__))
 #else
 #  define _LIBCPP_FALLTHROUGH() ((void)0)
@@ -1250,9 +1325,15 @@ _LIBCPP_FUNC_VIS extern "C" void __sanitizer_annotate_contiguous_container(
      __attribute__((availability(ios,strict,introduced=10.0)))                 \
      __attribute__((availability(tvos,strict,introduced=10.0)))                \
      __attribute__((availability(watchos,strict,introduced=3.0)))
-#  define _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS __attribute__((unavailable))
-#  define _LIBCPP_AVAILABILITY_BAD_ARRAY_LENGTH __attribute__((unavailable))
-#  define _LIBCPP_AVAILABILITY_BAD_ANY_CAST __attribute__((unavailable))
+#  define _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS                             \
+     __attribute__((availability(macosx,strict,introduced=10.14)))             \
+     __attribute__((availability(ios,strict,introduced=12.0)))                 \
+     __attribute__((availability(tvos,strict,introduced=12.0)))                \
+     __attribute__((availability(watchos,strict,introduced=5.0)))
+#  define _LIBCPP_AVAILABILITY_BAD_VARIANT_ACCESS                              \
+     _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS
+#  define _LIBCPP_AVAILABILITY_BAD_ANY_CAST                                    \
+     _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS
 #  define _LIBCPP_AVAILABILITY_UNCAUGHT_EXCEPTIONS                             \
      __attribute__((availability(macosx,strict,introduced=10.12)))             \
      __attribute__((availability(ios,strict,introduced=10.0)))                 \
@@ -1276,8 +1357,8 @@ _LIBCPP_FUNC_VIS extern "C" void __sanitizer_annotate_contiguous_container(
      __attribute__((availability(ios,strict,introduced=7.0)))
 #else
 #  define _LIBCPP_AVAILABILITY_SHARED_MUTEX
+#  define _LIBCPP_AVAILABILITY_BAD_VARIANT_ACCESS
 #  define _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS
-#  define _LIBCPP_AVAILABILITY_BAD_ARRAY_LENGTH
 #  define _LIBCPP_AVAILABILITY_BAD_ANY_CAST
 #  define _LIBCPP_AVAILABILITY_UNCAUGHT_EXCEPTIONS
 #  define _LIBCPP_AVAILABILITY_SIZED_NEW_DELETE
@@ -1289,26 +1370,30 @@ _LIBCPP_FUNC_VIS extern "C" void __sanitizer_annotate_contiguous_container(
 
 // Define availability that depends on _LIBCPP_NO_EXCEPTIONS.
 #ifdef _LIBCPP_NO_EXCEPTIONS
-#  define _LIBCPP_AVAILABILITY_DYNARRAY
 #  define _LIBCPP_AVAILABILITY_FUTURE
 #  define _LIBCPP_AVAILABILITY_THROW_BAD_ANY_CAST
+#  define _LIBCPP_AVAILABILITY_THROW_BAD_OPTIONAL_ACCESS
+#  define _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS
 #else
-#  define _LIBCPP_AVAILABILITY_DYNARRAY _LIBCPP_AVAILABILITY_BAD_ARRAY_LENGTH
-#  define _LIBCPP_AVAILABILITY_FUTURE _LIBCPP_AVAILABILITY_FUTURE_ERROR
-#  define _LIBCPP_AVAILABILITY_THROW_BAD_ANY_CAST                              \
-     _LIBCPP_AVAILABILITY_BAD_ANY_CAST
-#endif
-
-// Availability of stream API in the dylib got dropped and re-added.  The
-// extern template should effectively be available at:
-//    availability(macosx,introduced=10.9)
-//    availability(ios,introduced=7.0)
-#if defined(_LIBCPP_USE_AVAILABILITY_APPLE) &&                                 \
+#  define _LIBCPP_AVAILABILITY_FUTURE                    _LIBCPP_AVAILABILITY_FUTURE_ERROR
+#  define _LIBCPP_AVAILABILITY_THROW_BAD_ANY_CAST        _LIBCPP_AVAILABILITY_BAD_ANY_CAST
+#  define _LIBCPP_AVAILABILITY_THROW_BAD_OPTIONAL_ACCESS _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS
+#  define _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS  _LIBCPP_AVAILABILITY_BAD_VARIANT_ACCESS
+#endif
+
+// The stream API was dropped and re-added in the dylib shipped on macOS
+// and iOS. We can only assume the dylib to provide these definitions for
+// macosx >= 10.9 and ios >= 7.0. Otherwise, the definitions are available
+// from the headers, but not from the dylib. Explicit instantiation
+// declarations for streams exist conditionally to this; if we provide
+// an explicit instantiation declaration and we try to deploy to a dylib
+// that does not provide those symbols, we'll get a load-time error.
+#if !defined(_LIBCPP_BUILDING_LIBRARY) &&                                      \
     ((defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) &&                \
       __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ < 1090) ||                 \
      (defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__) &&               \
       __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ < 70000))
-#define _LIBCPP_AVAILABILITY_NO_STREAMS_EXTERN_TEMPLATE
+#  define _LIBCPP_DO_NOT_ASSUME_STREAMS_EXPLICIT_INSTANTIATION_IN_DYLIB
 #endif
 
 #if defined(_LIBCPP_COMPILER_IBM)
@@ -1351,6 +1436,8 @@ _LIBCPP_FUNC_VIS extern "C" void __sanitizer_annotate_contiguous_container(
 #  endif // defined(_LIBCPP_ABI_MICROSOFT) && !defined(_LIBCPP_BUILDING_LIBRARY)
 #endif // _LIBCPP_NO_AUTO_LINK
 
+#define _LIBCPP_UNUSED_VAR(x) ((void)(x))
+
 #endif // __cplusplus
 
 #endif // _LIBCPP_CONFIG
diff --git a/include/__config_site.in b/include/__config_site.in
index 8d980ff57cc8..580a6aa4c07b 100644
--- a/include/__config_site.in
+++ b/include/__config_site.in
@@ -14,6 +14,7 @@
 #cmakedefine _LIBCPP_ABI_UNSTABLE
 #cmakedefine _LIBCPP_ABI_FORCE_ITANIUM
 #cmakedefine _LIBCPP_ABI_FORCE_MICROSOFT
+#cmakedefine _LIBCPP_HIDE_FROM_ABI_PER_TU_BY_DEFAULT
 #cmakedefine _LIBCPP_HAS_NO_GLOBAL_FILESYSTEM_NAMESPACE
 #cmakedefine _LIBCPP_HAS_NO_STDIN
 #cmakedefine _LIBCPP_HAS_NO_STDOUT
@@ -27,6 +28,7 @@
 #cmakedefine _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL
 #cmakedefine _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS
 #cmakedefine _LIBCPP_NO_VCRUNTIME
+#cmakedefine _LIBCPP_ABI_NAMESPACE @_LIBCPP_ABI_NAMESPACE@
 
 @_LIBCPP_ABI_DEFINES@
 
diff --git a/include/__debug b/include/__debug
index d01bacdf7edc..a8788f68f8f4 100644
--- a/include/__debug
+++ b/include/__debug
@@ -74,7 +74,7 @@ typedef void(*__libcpp_debug_function_type)(__libcpp_debug_info const&);
 
 /// __libcpp_debug_function - The handler function called when a _LIBCPP_ASSERT
 ///    fails.
-extern _LIBCPP_EXTERN_VIS __libcpp_debug_function_type __libcpp_debug_function;
+extern _LIBCPP_EXPORTED_FROM_ABI __libcpp_debug_function_type __libcpp_debug_function;
 
 /// __libcpp_abort_debug_function - A debug handler that aborts when called.
 _LIBCPP_NORETURN _LIBCPP_FUNC_VIS
diff --git a/include/__functional_base b/include/__functional_base
index 57fdf2b9f663..032be99bf6a5 100644
--- a/include/__functional_base
+++ b/include/__functional_base
@@ -50,7 +50,7 @@ template <class _Tp>
 #endif
 struct _LIBCPP_TEMPLATE_VIS less : binary_function<_Tp, _Tp, bool>
 {
-    _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY 
+    _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY
     bool operator()(const _Tp& __x, const _Tp& __y) const
         {return __x < __y;}
 };
@@ -59,7 +59,7 @@ struct _LIBCPP_TEMPLATE_VIS less : binary_function<_Tp, _Tp, bool>
 template <>
 struct _LIBCPP_TEMPLATE_VIS less<void>
 {
-    template <class _T1, class _T2> 
+    template <class _T1, class _T2>
     _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY
     auto operator()(_T1&& __t, _T2&& __u) const
     _NOEXCEPT_(noexcept(_VSTD::forward<_T1>(__t) < _VSTD::forward<_T2>(__u)))
@@ -552,7 +552,7 @@ template <class _Tp, class, class = void>
 struct __is_transparent : false_type {};
 
 template <class _Tp, class _Up>
-struct __is_transparent<_Tp, _Up,  
+struct __is_transparent<_Tp, _Up,
                         typename __void_t<typename _Tp::is_transparent>::type>
    : true_type {};
 #endif
@@ -562,7 +562,7 @@ struct __is_transparent<_Tp, _Up,
 struct _LIBCPP_TEMPLATE_VIS allocator_arg_t { };
 
 #if defined(_LIBCPP_CXX03_LANG) || defined(_LIBCPP_BUILDING_LIBRARY)
-extern const allocator_arg_t allocator_arg;
+extern _LIBCPP_EXPORTED_FROM_ABI const allocator_arg_t allocator_arg;
 #else
 /* _LIBCPP_INLINE_VAR */ constexpr allocator_arg_t allocator_arg = allocator_arg_t();
 #endif
diff --git a/include/__hash_table b/include/__hash_table
index c77de961be6b..6f5b183105e6 100644
--- a/include/__hash_table
+++ b/include/__hash_table
@@ -35,15 +35,6 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 template <class _Key, class _Tp>
 struct __hash_value_type;
 
-template <class _Key, class _Cp, class _Hash,
-          bool =  is_empty<_Hash>::value && !__libcpp_is_final<_Hash>::value>
-class __unordered_map_hasher;
-
-template <class _Key, class _Cp, class _Pred,
-          bool = is_empty<_Pred>::value && !__libcpp_is_final<_Pred>::value
-         >
-class __unordered_map_equal;
-
 #ifndef _LIBCPP_CXX03_LANG
 template <class _Tp>
 struct __is_hash_value_type_imp : false_type {};
@@ -418,7 +409,7 @@ public:
         _LIBCPP_DEBUG_MODE(__get_db()->__insert_i(this));
     }
 
-    _LIBCPP_INLINE_VISIBILITY 
+    _LIBCPP_INLINE_VISIBILITY
     __hash_const_iterator(const __non_const_iterator& __x) _NOEXCEPT
         : __node_(__x.__node_)
     {
@@ -871,35 +862,32 @@ struct __generic_container_node_destructor<__hash_node<_Tp, _VoidPtr>, _Alloc>
 };
 #endif
 
+template <class _Key, class _Hash, class _Equal>
+struct __enforce_unordered_container_requirements {
 #ifndef _LIBCPP_CXX03_LANG
-template <class _Key, class _Hash, class _Equal, class _Alloc>
-struct __diagnose_hash_table_helper {
-  static constexpr bool __trigger_diagnostics()
-    _LIBCPP_DIAGNOSE_WARNING(__check_hash_requirements<_Key, _Hash>::value
-                         && !__invokable<_Hash const&, _Key const&>::value,
-      "the specified hash functor does not provide a const call operator")
-    _LIBCPP_DIAGNOSE_WARNING(is_copy_constructible<_Equal>::value
-                          && !__invokable<_Equal const&, _Key const&, _Key const&>::value,
-      "the specified comparator type does not provide a const call operator")
-  {
     static_assert(__check_hash_requirements<_Key, _Hash>::value,
-      "the specified hash does not meet the Hash requirements");
+    "the specified hash does not meet the Hash requirements");
     static_assert(is_copy_constructible<_Equal>::value,
-      "the specified comparator is required to be copy constructible");
-    return true;
-  }
+    "the specified comparator is required to be copy constructible");
+#endif
+    typedef int type;
 };
 
-template <class _Key, class _Value, class _Hash, class _Equal, class _Alloc>
-struct __diagnose_hash_table_helper<
-  __hash_value_type<_Key, _Value>,
-  __unordered_map_hasher<_Key, __hash_value_type<_Key, _Value>, _Hash>,
-  __unordered_map_equal<_Key, __hash_value_type<_Key, _Value>, _Equal>,
-  _Alloc>
-: __diagnose_hash_table_helper<_Key, _Hash, _Equal, _Alloc>
-{
-};
-#endif // _LIBCPP_CXX03_LANG
+template <class _Key, class _Hash, class _Equal>
+#ifndef _LIBCPP_CXX03_LANG
+    _LIBCPP_DIAGNOSE_WARNING(!__invokable<_Equal const&, _Key const&, _Key const&>::value,
+    "the specified comparator type does not provide a const call operator")
+    _LIBCPP_DIAGNOSE_WARNING(!__invokable<_Hash const&, _Key const&>::value,
+    "the specified hash functor does not provide a const call operator")
+#endif
+typename __enforce_unordered_container_requirements<_Key, _Hash, _Equal>::type
+__diagnose_unordered_container_requirements(int);
+
+// This dummy overload is used so that the compiler won't emit a spurious
+// "no matching function for call to __diagnose_unordered_xxx" diagnostic
+// when the overload above causes a hard error.
+template <class _Key, class _Hash, class _Equal>
+int __diagnose_unordered_container_requirements(void*);
 
 template <class _Tp, class _Hash, class _Equal, class _Alloc>
 class __hash_table
@@ -963,10 +951,6 @@ private:
     typedef allocator_traits<__pointer_allocator>          __pointer_alloc_traits;
     typedef typename __bucket_list_deleter::pointer       __node_pointer_pointer;
 
-#ifndef _LIBCPP_CXX03_LANG
-    static_assert(__diagnose_hash_table_helper<_Tp, _Hash, _Equal, _Alloc>::__trigger_diagnostics(), "");
-#endif
-
     // --- Member data begin ---
     __bucket_list                                         __bucket_list_;
     __compressed_pair<__first_node, __node_allocator>     __p1_;
@@ -1058,8 +1042,26 @@ public:
         );
     }
 
+private:
+    _LIBCPP_INLINE_VISIBILITY
+    __next_pointer __node_insert_multi_prepare(size_t __cp_hash,
+                                               value_type& __cp_val);
+    _LIBCPP_INLINE_VISIBILITY
+    void __node_insert_multi_perform(__node_pointer __cp,
+                                     __next_pointer __pn) _NOEXCEPT;
+
+    _LIBCPP_INLINE_VISIBILITY
+    __next_pointer __node_insert_unique_prepare(size_t __nd_hash,
+                                                value_type& __nd_val);
+    _LIBCPP_INLINE_VISIBILITY
+    void __node_insert_unique_perform(__node_pointer __ptr) _NOEXCEPT;
+
+public:
+    _LIBCPP_INLINE_VISIBILITY
     pair<iterator, bool> __node_insert_unique(__node_pointer __nd);
+    _LIBCPP_INLINE_VISIBILITY
     iterator             __node_insert_multi(__node_pointer __nd);
+    _LIBCPP_INLINE_VISIBILITY
     iterator             __node_insert_multi(const_iterator __p,
                                              __node_pointer __nd);
 
@@ -1170,6 +1172,9 @@ public:
     _LIBCPP_INLINE_VISIBILITY
     iterator __node_handle_insert_unique(const_iterator __hint,
                                          _NodeHandle&& __nh);
+    template <class _Table>
+    _LIBCPP_INLINE_VISIBILITY
+    void __node_handle_merge_unique(_Table& __source);
 
     template <class _NodeHandle>
     _LIBCPP_INLINE_VISIBILITY
@@ -1177,6 +1182,9 @@ public:
     template <class _NodeHandle>
     _LIBCPP_INLINE_VISIBILITY
     iterator __node_handle_insert_multi(const_iterator __hint, _NodeHandle&& __nh);
+    template <class _Table>
+    _LIBCPP_INLINE_VISIBILITY
+    void __node_handle_merge_multi(_Table& __source);
 
     template <class _NodeHandle>
     _LIBCPP_INLINE_VISIBILITY
@@ -1849,73 +1857,112 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>::clear() _NOEXCEPT
     }
 }
 
+
+// Prepare the container for an insertion of the value __value with the hash
+// __hash. This does a lookup into the container to see if __value is already
+// present, and performs a rehash if necessary. Returns a pointer to the
+// existing element if it exists, otherwise nullptr.
+//
+// Note that this function does forward exceptions if key_eq() throws, and never
+// mutates __value or actually inserts into the map.
 template <class _Tp, class _Hash, class _Equal, class _Alloc>
-pair<typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::iterator, bool>
-__hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_insert_unique(__node_pointer __nd)
+_LIBCPP_INLINE_VISIBILITY
+typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::__next_pointer
+__hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_insert_unique_prepare(
+    size_t __hash, value_type& __value)
 {
-    __nd->__hash_ = hash_function()(__nd->__value_);
     size_type __bc = bucket_count();
-    bool __inserted = false;
-    __next_pointer __ndptr;
-    size_t __chash;
+
     if (__bc != 0)
     {
-        __chash = __constrain_hash(__nd->__hash_, __bc);
-        __ndptr = __bucket_list_[__chash];
+        size_t __chash = __constrain_hash(__hash, __bc);
+        __next_pointer __ndptr = __bucket_list_[__chash];
         if (__ndptr != nullptr)
         {
             for (__ndptr = __ndptr->__next_; __ndptr != nullptr &&
                                              __constrain_hash(__ndptr->__hash(), __bc) == __chash;
                                                      __ndptr = __ndptr->__next_)
             {
-                if (key_eq()(__ndptr->__upcast()->__value_, __nd->__value_))
-                    goto __done;
+                if (key_eq()(__ndptr->__upcast()->__value_, __value))
+                    return __ndptr;
             }
         }
     }
+    if (size()+1 > __bc * max_load_factor() || __bc == 0)
     {
-        if (size()+1 > __bc * max_load_factor() || __bc == 0)
-        {
-            rehash(_VSTD::max<size_type>(2 * __bc + !__is_hash_power2(__bc),
-                           size_type(ceil(float(size() + 1) / max_load_factor()))));
-            __bc = bucket_count();
-            __chash = __constrain_hash(__nd->__hash_, __bc);
-        }
-        // insert_after __bucket_list_[__chash], or __first_node if bucket is null
-        __next_pointer __pn = __bucket_list_[__chash];
-        if (__pn == nullptr)
-        {
-            __pn =__p1_.first().__ptr();
-            __nd->__next_ = __pn->__next_;
-            __pn->__next_ = __nd->__ptr();
-            // fix up __bucket_list_
-            __bucket_list_[__chash] = __pn;
-            if (__nd->__next_ != nullptr)
-                __bucket_list_[__constrain_hash(__nd->__next_->__hash(), __bc)] = __nd->__ptr();
-        }
-        else
-        {
-            __nd->__next_ = __pn->__next_;
-            __pn->__next_ = __nd->__ptr();
-        }
-        __ndptr = __nd->__ptr();
-        // increment size
-        ++size();
+        rehash(_VSTD::max<size_type>(2 * __bc + !__is_hash_power2(__bc),
+                                     size_type(ceil(float(size() + 1) / max_load_factor()))));
+    }
+    return nullptr;
+}
+
+// Insert the node __nd into the container by pushing it into the right bucket,
+// and updating size(). Assumes that __nd->__hash is up-to-date, and that
+// rehashing has already occurred and that no element with the same key exists
+// in the map.
+template <class _Tp, class _Hash, class _Equal, class _Alloc>
+_LIBCPP_INLINE_VISIBILITY
+void
+__hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_insert_unique_perform(
+    __node_pointer __nd) _NOEXCEPT
+{
+    size_type __bc = bucket_count();
+    size_t __chash = __constrain_hash(__nd->__hash(), __bc);
+    // insert_after __bucket_list_[__chash], or __first_node if bucket is null
+    __next_pointer __pn = __bucket_list_[__chash];
+    if (__pn == nullptr)
+    {
+        __pn =__p1_.first().__ptr();
+        __nd->__next_ = __pn->__next_;
+        __pn->__next_ = __nd->__ptr();
+        // fix up __bucket_list_
+        __bucket_list_[__chash] = __pn;
+        if (__nd->__next_ != nullptr)
+            __bucket_list_[__constrain_hash(__nd->__next_->__hash(), __bc)] = __nd->__ptr();
+    }
+    else
+    {
+        __nd->__next_ = __pn->__next_;
+        __pn->__next_ = __nd->__ptr();
+    }
+    ++size();
+}
+
+template <class _Tp, class _Hash, class _Equal, class _Alloc>
+pair<typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::iterator, bool>
+__hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_insert_unique(__node_pointer __nd)
+{
+    __nd->__hash_ = hash_function()(__nd->__value_);
+    __next_pointer __existing_node =
+        __node_insert_unique_prepare(__nd->__hash(), __nd->__value_);
+
+    // Insert the node, unless it already exists in the container.
+    bool __inserted = false;
+    if (__existing_node == nullptr)
+    {
+        __node_insert_unique_perform(__nd);
+        __existing_node = __nd->__ptr();
         __inserted = true;
     }
-__done:
 #if _LIBCPP_DEBUG_LEVEL >= 2
-    return pair<iterator, bool>(iterator(__ndptr, this), __inserted);
+    return pair<iterator, bool>(iterator(__existing_node, this), __inserted);
 #else
-    return pair<iterator, bool>(iterator(__ndptr), __inserted);
+    return pair<iterator, bool>(iterator(__existing_node), __inserted);
 #endif
 }
 
+// Prepare the container for an insertion of the value __cp_val with the hash
+// __cp_hash. This does a lookup into the container to see if __cp_value is
+// already present, and performs a rehash if necessary. Returns a pointer to the
+// last occurance of __cp_val in the map.
+//
+// Note that this function does forward exceptions if key_eq() throws, and never
+// mutates __value or actually inserts into the map.
 template <class _Tp, class _Hash, class _Equal, class _Alloc>
-typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::iterator
-__hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_insert_multi(__node_pointer __cp)
+typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::__next_pointer
+__hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_insert_multi_prepare(
+    size_t __cp_hash, value_type& __cp_val)
 {
-    __cp->__hash_ = hash_function()(__cp->__value_);
     size_type __bc = bucket_count();
     if (size()+1 > __bc * max_load_factor() || __bc == 0)
     {
@@ -1923,20 +1970,9 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_insert_multi(__node_pointer __c
                        size_type(ceil(float(size() + 1) / max_load_factor()))));
         __bc = bucket_count();
     }
-    size_t __chash = __constrain_hash(__cp->__hash_, __bc);
+    size_t __chash = __constrain_hash(__cp_hash, __bc);
     __next_pointer __pn = __bucket_list_[__chash];
-    if (__pn == nullptr)
-    {
-        __pn =__p1_.first().__ptr();
-        __cp->__next_ = __pn->__next_;
-        __pn->__next_ = __cp->__ptr();
-        // fix up __bucket_list_
-        __bucket_list_[__chash] = __pn;
-        if (__cp->__next_ != nullptr)
-            __bucket_list_[__constrain_hash(__cp->__next_->__hash(), __bc)]
-                = __cp->__ptr();
-    }
-    else
+    if (__pn != nullptr)
     {
         for (bool __found = false; __pn->__next_ != nullptr &&
                                    __constrain_hash(__pn->__next_->__hash(), __bc) == __chash;
@@ -1947,8 +1983,8 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_insert_multi(__node_pointer __c
             //      true        true        loop
             //      false       true        set __found to true
             //      true        false       break
-            if (__found != (__pn->__next_->__hash() == __cp->__hash_ &&
-                            key_eq()(__pn->__next_->__upcast()->__value_, __cp->__value_)))
+            if (__found != (__pn->__next_->__hash() == __cp_hash &&
+                            key_eq()(__pn->__next_->__upcast()->__value_, __cp_val)))
             {
                 if (!__found)
                     __found = true;
@@ -1956,6 +1992,35 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_insert_multi(__node_pointer __c
                     break;
             }
         }
+    }
+    return __pn;
+}
+
+// Insert the node __cp into the container after __pn (which is the last node in
+// the bucket that compares equal to __cp). Rehashing, and checking for
+// uniqueness has already been performed (in __node_insert_multi_prepare), so
+// all we need to do is update the bucket and size(). Assumes that __cp->__hash
+// is up-to-date.
+template <class _Tp, class _Hash, class _Equal, class _Alloc>
+void
+__hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_insert_multi_perform(
+    __node_pointer __cp, __next_pointer __pn) _NOEXCEPT
+{
+    size_type __bc = bucket_count();
+    size_t __chash = __constrain_hash(__cp->__hash_, __bc);
+    if (__pn == nullptr)
+    {
+        __pn =__p1_.first().__ptr();
+        __cp->__next_ = __pn->__next_;
+        __pn->__next_ = __cp->__ptr();
+        // fix up __bucket_list_
+        __bucket_list_[__chash] = __pn;
+        if (__cp->__next_ != nullptr)
+            __bucket_list_[__constrain_hash(__cp->__next_->__hash(), __bc)]
+                = __cp->__ptr();
+    }
+    else
+    {
         __cp->__next_ = __pn->__next_;
         __pn->__next_ = __cp->__ptr();
         if (__cp->__next_ != nullptr)
@@ -1966,6 +2031,17 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_insert_multi(__node_pointer __c
         }
     }
     ++size();
+}
+
+
+template <class _Tp, class _Hash, class _Equal, class _Alloc>
+typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::iterator
+__hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_insert_multi(__node_pointer __cp)
+{
+    __cp->__hash_ = hash_function()(__cp->__value_);
+    __next_pointer __pn = __node_insert_multi_prepare(__cp->__hash(), __cp->__value_);
+    __node_insert_multi_perform(__cp, __pn);
+
 #if _LIBCPP_DEBUG_LEVEL >= 2
     return iterator(__cp->__ptr(), this);
 #else
@@ -2217,6 +2293,32 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_handle_extract(
 }
 
 template <class _Tp, class _Hash, class _Equal, class _Alloc>
+template <class _Table>
+_LIBCPP_INLINE_VISIBILITY
+void
+__hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_handle_merge_unique(
+    _Table& __source)
+{
+    static_assert(is_same<__node, typename _Table::__node>::value, "");
+
+    for (typename _Table::iterator __it = __source.begin();
+         __it != __source.end();)
+    {
+        __node_pointer __src_ptr = __it.__node_->__upcast();
+        size_t __hash = hash_function()(__src_ptr->__value_);
+        __next_pointer __existing_node =
+            __node_insert_unique_prepare(__hash, __src_ptr->__value_);
+        auto __prev_iter = __it++;
+        if (__existing_node == nullptr)
+        {
+            (void)__source.remove(__prev_iter).release();
+            __src_ptr->__hash_ = __hash;
+            __node_insert_unique_perform(__src_ptr);
+        }
+    }
+}
+
+template <class _Tp, class _Hash, class _Equal, class _Alloc>
 template <class _NodeHandle>
 _LIBCPP_INLINE_VISIBILITY
 typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::iterator
@@ -2244,6 +2346,27 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_handle_insert_multi(
     return __result;
 }
 
+template <class _Tp, class _Hash, class _Equal, class _Alloc>
+template <class _Table>
+_LIBCPP_INLINE_VISIBILITY
+void
+__hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_handle_merge_multi(
+    _Table& __source)
+{
+    static_assert(is_same<typename _Table::__node, __node>::value, "");
+
+    for (typename _Table::iterator __it = __source.begin();
+         __it != __source.end();)
+    {
+        __node_pointer __src_ptr = __it.__node_->__upcast();
+        size_t __src_hash = hash_function()(__src_ptr->__value_);
+        __next_pointer __pn =
+            __node_insert_multi_prepare(__src_hash, __src_ptr->__value_);
+        (void)__source.remove(__it++).release();
+        __src_ptr->__hash_ = __src_hash;
+        __node_insert_multi_perform(__src_ptr, __pn);
+    }
+}
 #endif  // _LIBCPP_STD_VER > 14
 
 template <class _Tp, class _Hash, class _Equal, class _Alloc>
diff --git a/include/__libcpp_version b/include/__libcpp_version
index 2bdc6533bec7..e002b3628b34 100644
--- a/include/__libcpp_version
+++ b/include/__libcpp_version
@@ -1 +1 @@
-7000
+8000
diff --git a/include/__locale b/include/__locale
index f43e7b4303d3..cde9cc85f6b5 100644
--- a/include/__locale
+++ b/include/__locale
@@ -1255,13 +1255,13 @@ struct __narrow_to_utf8<8>
 };
 
 template <>
-struct __narrow_to_utf8<16>
+struct _LIBCPP_TEMPLATE_VIS __narrow_to_utf8<16>
     : public codecvt<char16_t, char, mbstate_t>
 {
     _LIBCPP_INLINE_VISIBILITY
     __narrow_to_utf8() : codecvt<char16_t, char, mbstate_t>(1) {}
 
-    ~__narrow_to_utf8();
+    _LIBCPP_EXPORTED_FROM_ABI ~__narrow_to_utf8();
 
     template <class _OutputIterator, class _CharT>
     _LIBCPP_INLINE_VISIBILITY
@@ -1289,13 +1289,13 @@ struct __narrow_to_utf8<16>
 };
 
 template <>
-struct __narrow_to_utf8<32>
+struct _LIBCPP_TEMPLATE_VIS __narrow_to_utf8<32>
     : public codecvt<char32_t, char, mbstate_t>
 {
     _LIBCPP_INLINE_VISIBILITY
     __narrow_to_utf8() : codecvt<char32_t, char, mbstate_t>(1) {}
 
-    ~__narrow_to_utf8();
+    _LIBCPP_EXPORTED_FROM_ABI ~__narrow_to_utf8();
 
     template <class _OutputIterator, class _CharT>
     _LIBCPP_INLINE_VISIBILITY
@@ -1345,13 +1345,13 @@ struct __widen_from_utf8<8>
 };
 
 template <>
-struct __widen_from_utf8<16>
+struct _LIBCPP_TEMPLATE_VIS __widen_from_utf8<16>
     : public codecvt<char16_t, char, mbstate_t>
 {
     _LIBCPP_INLINE_VISIBILITY
     __widen_from_utf8() : codecvt<char16_t, char, mbstate_t>(1) {}
 
-    ~__widen_from_utf8();
+    _LIBCPP_EXPORTED_FROM_ABI ~__widen_from_utf8();
 
     template <class _OutputIterator>
     _LIBCPP_INLINE_VISIBILITY
@@ -1379,13 +1379,13 @@ struct __widen_from_utf8<16>
 };
 
 template <>
-struct __widen_from_utf8<32>
+struct _LIBCPP_TEMPLATE_VIS __widen_from_utf8<32>
     : public codecvt<char32_t, char, mbstate_t>
 {
     _LIBCPP_INLINE_VISIBILITY
     __widen_from_utf8() : codecvt<char32_t, char, mbstate_t>(1) {}
 
-    ~__widen_from_utf8();
+    _LIBCPP_EXPORTED_FROM_ABI ~__widen_from_utf8();
 
     template <class _OutputIterator>
     _LIBCPP_INLINE_VISIBILITY
diff --git a/include/__mutex_base b/include/__mutex_base
index 4659ca9298c9..da21a5f8eb6a 100644
--- a/include/__mutex_base
+++ b/include/__mutex_base
@@ -76,9 +76,9 @@ struct _LIBCPP_TYPE_VIS adopt_lock_t {};
 
 #if defined(_LIBCPP_CXX03_LANG) || defined(_LIBCPP_BUILDING_LIBRARY)
 
-extern const defer_lock_t  defer_lock;
-extern const try_to_lock_t try_to_lock;
-extern const adopt_lock_t  adopt_lock;
+extern _LIBCPP_EXPORTED_FROM_ABI const defer_lock_t  defer_lock;
+extern _LIBCPP_EXPORTED_FROM_ABI const try_to_lock_t try_to_lock;
+extern _LIBCPP_EXPORTED_FROM_ABI const adopt_lock_t  adopt_lock;
 
 #else
 
diff --git a/include/__node_handle b/include/__node_handle
index fe09f3c1e51c..a9cf3b7217a3 100644
--- a/include/__node_handle
+++ b/include/__node_handle
@@ -26,8 +26,6 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 14
 
-#define __cpp_lib_node_extract 201606L
-
 // Specialized in __tree & __hash_table for their _NodeType.
 template <class _NodeType, class _Alloc>
 struct __generic_container_node_destructor;
diff --git a/include/__sso_allocator b/include/__sso_allocator
index 40027363a185..8aca0495d756 100644
--- a/include/__sso_allocator
+++ b/include/__sso_allocator
@@ -55,14 +55,14 @@ public:
             __allocated_ = true;
             return (pointer)&buf_;
         }
-        return static_cast<pointer>(_VSTD::__libcpp_allocate(__n * sizeof(_Tp), __alignof(_Tp)));
+        return static_cast<pointer>(_VSTD::__libcpp_allocate(__n * sizeof(_Tp), _LIBCPP_ALIGNOF(_Tp)));
     }
-    _LIBCPP_INLINE_VISIBILITY void deallocate(pointer __p, size_type)
+    _LIBCPP_INLINE_VISIBILITY void deallocate(pointer __p, size_type __n)
     {
         if (__p == (pointer)&buf_)
             __allocated_ = false;
         else
-            _VSTD::__libcpp_deallocate(__p, __alignof(_Tp));
+            _VSTD::__libcpp_deallocate(__p, __n * sizeof(_Tp), _LIBCPP_ALIGNOF(_Tp));
     }
     _LIBCPP_INLINE_VISIBILITY size_type max_size() const throw() {return size_type(~0) / sizeof(_Tp);}
 
diff --git a/include/__string b/include/__string
index 44c55987f9f7..1ddeec7149fb 100644
--- a/include/__string
+++ b/include/__string
@@ -47,6 +47,7 @@ struct char_traits
 
 template <> struct char_traits<char>;
 template <> struct char_traits<wchar_t>;
+template <> struct char_traits<char8_t>;  // c++20
 
 }  // std
 
@@ -389,6 +390,102 @@ char_traits<wchar_t>::find(const char_type* __s, size_t __n, const char_type& __
 }
 
 
+#ifndef _LIBCPP_NO_HAS_CHAR8_T
+
+template <>
+struct _LIBCPP_TEMPLATE_VIS char_traits<char8_t>
+{
+    typedef char8_t        char_type;
+    typedef unsigned int   int_type;
+    typedef streamoff      off_type;
+    typedef u8streampos    pos_type;
+    typedef mbstate_t      state_type;
+
+    static inline constexpr void assign(char_type& __c1, const char_type& __c2) noexcept
+        {__c1 = __c2;}
+    static inline constexpr bool eq(char_type __c1, char_type __c2) noexcept
+        {return __c1 == __c2;}
+    static inline constexpr bool lt(char_type __c1, char_type __c2) noexcept
+        {return __c1 < __c2;}
+
+    static constexpr
+    int              compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
+
+    static constexpr
+    size_t           length(const char_type* __s) _NOEXCEPT;
+ 
+    _LIBCPP_INLINE_VISIBILITY static constexpr
+    const char_type* find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT;
+ 
+    static char_type*       move(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
+        {return __n == 0 ? __s1 : (char_type*) memmove(__s1, __s2, __n);}
+ 
+    static char_type*       copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
+       {
+            _LIBCPP_ASSERT(__s2 < __s1 || __s2 >= __s1+__n, "char_traits::copy overlapped range");
+            return __n == 0 ? __s1 : (char_type*)memcpy(__s1, __s2, __n);
+       }
+ 
+    static char_type*       assign(char_type* __s, size_t __n, char_type __a) _NOEXCEPT
+        {return __n == 0 ? __s : (char_type*)memset(__s, to_int_type(__a), __n);}
+
+    static inline constexpr int_type  not_eof(int_type __c) noexcept
+        {return eq_int_type(__c, eof()) ? ~eof() : __c;}
+    static inline constexpr char_type to_char_type(int_type __c) noexcept
+        {return char_type(__c);}
+    static inline constexpr int_type to_int_type(char_type __c) noexcept
+        {return int_type(__c);}
+    static inline constexpr bool eq_int_type(int_type __c1, int_type __c2) noexcept
+        {return __c1 == __c2;}
+    static inline constexpr int_type eof() noexcept
+        {return int_type(EOF);}
+};
+
+// TODO use '__builtin_strlen' if it ever supports char8_t ??
+inline constexpr
+size_t
+char_traits<char8_t>::length(const char_type* __s) _NOEXCEPT
+{
+    size_t __len = 0;
+    for (; !eq(*__s, char_type(0)); ++__s)
+        ++__len;
+    return __len;
+}
+
+inline constexpr
+int
+char_traits<char8_t>::compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
+{
+#if __has_feature(cxx_constexpr_string_builtins)
+    return __builtin_memcmp(__s1, __s2, __n);
+#else
+    for (; __n; --__n, ++__s1, ++__s2)
+    {
+        if (lt(*__s1, *__s2))
+            return -1;
+        if (lt(*__s2, *__s1))
+            return 1;
+    }
+    return 0;
+#endif
+}
+
+// TODO use '__builtin_char_memchr' if it ever supports char8_t ??
+inline constexpr
+const char8_t*
+char_traits<char8_t>::find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT
+{
+    for (; __n; --__n)
+    {
+        if (eq(*__s, __a))
+            return __s;
+        ++__s;
+    }
+    return 0;
+}
+
+#endif // #_LIBCPP_NO_HAS_CHAR8_T
+
 #ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
 
 template <>
diff --git a/include/__threading_support b/include/__threading_support
index 3c1eff22f530..202490062729 100644
--- a/include/__threading_support
+++ b/include/__threading_support
@@ -70,7 +70,7 @@ typedef pthread_t __libcpp_thread_id;
 
 typedef pthread_t __libcpp_thread_t;
 
-// Thrad Local Storage
+// Thread Local Storage
 typedef pthread_key_t __libcpp_tls_key;
 
 #define _LIBCPP_TLS_DESTRUCTOR_CC
diff --git a/include/__tree b/include/__tree
index af9b9616df8a..814851085b98 100644
--- a/include/__tree
+++ b/include/__tree
@@ -40,10 +40,6 @@ template <class _Tp, class _VoidPtr> class __tree_node;
 template <class _Key, class _Value>
 struct __value_type;
 
-template <class _Key, class _CP, class _Compare,
-    bool = is_empty<_Compare>::value && !__libcpp_is_final<_Compare>::value>
-class __map_value_compare;
-
 template <class _Allocator> class __map_node_destructor;
 template <class _TreeIterator> class _LIBCPP_TEMPLATE_VIS __map_iterator;
 template <class _TreeIterator> class _LIBCPP_TEMPLATE_VIS __map_const_iterator;
@@ -857,7 +853,7 @@ public:
     __tree_iterator operator--(int)
         {__tree_iterator __t(*this); --(*this); return __t;}
 
-    friend _LIBCPP_INLINE_VISIBILITY 
+    friend _LIBCPP_INLINE_VISIBILITY
         bool operator==(const __tree_iterator& __x, const __tree_iterator& __y)
         {return __x.__ptr_ == __y.__ptr_;}
     friend _LIBCPP_INLINE_VISIBILITY
@@ -966,24 +962,12 @@ private:
 
 };
 
+template<class _Tp, class _Compare>
 #ifndef _LIBCPP_CXX03_LANG
-template <class _Tp, class _Compare, class _Allocator>
-struct __diagnose_tree_helper {
-  static constexpr bool __trigger_diagnostics()
-      _LIBCPP_DIAGNOSE_WARNING(!__invokable<_Compare const&, _Tp const&, _Tp const&>::value,
-            "the specified comparator type does not provide a const call operator")
-  { return true; }
-};
-
-template <class _Key, class _Value, class _KeyComp, class _Alloc>
-struct __diagnose_tree_helper<
-    __value_type<_Key, _Value>,
-    __map_value_compare<_Key, __value_type<_Key, _Value>, _KeyComp>,
-    _Alloc
-> : __diagnose_tree_helper<_Key, _KeyComp, _Alloc>
-{
-};
-#endif // !_LIBCPP_CXX03_LANG
+    _LIBCPP_DIAGNOSE_WARNING(!std::__invokable<_Compare const&, _Tp const&, _Tp const&>::value,
+        "the specified comparator type does not provide a const call operator")
+#endif
+int __diagnose_non_const_comparator();
 
 template <class _Tp, class _Compare, class _Allocator>
 class __tree
@@ -1341,15 +1325,20 @@ public:
 
 #endif // !_LIBCPP_CXX03_LANG
 
+    _LIBCPP_INLINE_VISIBILITY
     pair<iterator, bool> __node_insert_unique(__node_pointer __nd);
+    _LIBCPP_INLINE_VISIBILITY
     iterator             __node_insert_unique(const_iterator __p,
                                               __node_pointer __nd);
 
+    _LIBCPP_INLINE_VISIBILITY
     iterator __node_insert_multi(__node_pointer __nd);
+    _LIBCPP_INLINE_VISIBILITY
     iterator __node_insert_multi(const_iterator __p, __node_pointer __nd);
 
 
-    _LIBCPP_INLINE_VISIBILITY iterator __remove_node_pointer(__node_pointer);
+    _LIBCPP_INLINE_VISIBILITY iterator
+    __remove_node_pointer(__node_pointer) _NOEXCEPT;
 
 #if _LIBCPP_STD_VER > 14
     template <class _NodeHandle, class _InsertReturnType>
@@ -1358,6 +1347,9 @@ public:
     template <class _NodeHandle>
     _LIBCPP_INLINE_VISIBILITY
     iterator __node_handle_insert_unique(const_iterator, _NodeHandle&&);
+    template <class _Tree>
+    _LIBCPP_INLINE_VISIBILITY
+    void __node_handle_merge_unique(_Tree& __source);
 
     template <class _NodeHandle>
     _LIBCPP_INLINE_VISIBILITY
@@ -1365,6 +1357,9 @@ public:
     template <class _NodeHandle>
     _LIBCPP_INLINE_VISIBILITY
     iterator __node_handle_insert_multi(const_iterator, _NodeHandle&&);
+    template <class _Tree>
+    _LIBCPP_INLINE_VISIBILITY
+    void __node_handle_merge_multi(_Tree& __source);
 
 
     template <class _NodeHandle>
@@ -1384,7 +1379,7 @@ public:
 
     void __insert_node_at(__parent_pointer     __parent,
                           __node_base_pointer& __child,
-                          __node_base_pointer __new_node);
+                          __node_base_pointer __new_node) _NOEXCEPT;
 
     template <class _Key>
         iterator find(const _Key& __v);
@@ -1488,7 +1483,7 @@ private:
     void __copy_assign_alloc(const __tree& __t, true_type)
         {
         if (__node_alloc() != __t.__node_alloc())
-        	clear();
+            clear();
         __node_alloc() = __t.__node_alloc();
         }
     _LIBCPP_INLINE_VISIBILITY
@@ -1830,7 +1825,7 @@ __tree<_Tp, _Compare, _Allocator>::operator=(__tree&& __t)
         __node_traits::propagate_on_container_move_assignment::value &&
         is_nothrow_move_assignable<value_compare>::value &&
         is_nothrow_move_assignable<__node_allocator>::value)
-        
+
 {
     __move_assign(__t, integral_constant<bool,
                   __node_traits::propagate_on_container_move_assignment::value>());
@@ -1844,10 +1839,6 @@ __tree<_Tp, _Compare, _Allocator>::~__tree()
 {
     static_assert((is_copy_constructible<value_compare>::value),
                  "Comparator must be copy-constructible.");
-#ifndef _LIBCPP_CXX03_LANG
-    static_assert((__diagnose_tree_helper<_Tp, _Compare, _Allocator>::
-                     __trigger_diagnostics()), "");
-#endif
   destroy(__root());
 }
 
@@ -2129,10 +2120,9 @@ __tree<_Tp, _Compare, _Allocator>::__find_equal(const_iterator __hint,
 }
 
 template <class _Tp, class _Compare, class _Allocator>
-void
-__tree<_Tp, _Compare, _Allocator>::__insert_node_at(__parent_pointer     __parent,
-                                                    __node_base_pointer& __child,
-                                                    __node_base_pointer  __new_node)
+void __tree<_Tp, _Compare, _Allocator>::__insert_node_at(
+    __parent_pointer __parent, __node_base_pointer& __child,
+    __node_base_pointer __new_node) _NOEXCEPT
 {
     __new_node->__left_   = nullptr;
     __new_node->__right_  = nullptr;
@@ -2384,7 +2374,7 @@ __tree<_Tp, _Compare, _Allocator>::__node_insert_multi(const_iterator __p,
 
 template <class _Tp, class _Compare, class _Allocator>
 typename __tree<_Tp, _Compare, _Allocator>::iterator
-__tree<_Tp, _Compare, _Allocator>::__remove_node_pointer(__node_pointer __ptr)
+__tree<_Tp, _Compare, _Allocator>::__remove_node_pointer(__node_pointer __ptr) _NOEXCEPT
 {
     iterator __r(__ptr);
     ++__r;
@@ -2472,6 +2462,30 @@ __tree<_Tp, _Compare, _Allocator>::__node_handle_extract(const_iterator __p)
 }
 
 template <class _Tp, class _Compare, class _Allocator>
+template <class _Tree>
+_LIBCPP_INLINE_VISIBILITY
+void
+__tree<_Tp, _Compare, _Allocator>::__node_handle_merge_unique(_Tree& __source)
+{
+    static_assert(is_same<typename _Tree::__node_pointer, __node_pointer>::value, "");
+
+    for (typename _Tree::iterator __i = __source.begin();
+         __i != __source.end();)
+    {
+        __node_pointer __src_ptr = __i.__get_np();
+        __parent_pointer __parent;
+        __node_base_pointer& __child =
+            __find_equal(__parent, _NodeTypes::__get_key(__src_ptr->__value_));
+        ++__i;
+        if (__child != nullptr)
+            continue;
+        __source.__remove_node_pointer(__src_ptr);
+        __insert_node_at(__parent, __child,
+                         static_cast<__node_base_pointer>(__src_ptr));
+    }
+}
+
+template <class _Tp, class _Compare, class _Allocator>
 template <class _NodeHandle>
 _LIBCPP_INLINE_VISIBILITY
 typename __tree<_Tp, _Compare, _Allocator>::iterator
@@ -2507,6 +2521,28 @@ __tree<_Tp, _Compare, _Allocator>::__node_handle_insert_multi(
     return iterator(__ptr);
 }
 
+template <class _Tp, class _Compare, class _Allocator>
+template <class _Tree>
+_LIBCPP_INLINE_VISIBILITY
+void
+__tree<_Tp, _Compare, _Allocator>::__node_handle_merge_multi(_Tree& __source)
+{
+    static_assert(is_same<typename _Tree::__node_pointer, __node_pointer>::value, "");
+
+    for (typename _Tree::iterator __i = __source.begin();
+         __i != __source.end();)
+    {
+        __node_pointer __src_ptr = __i.__get_np();
+        __parent_pointer __parent;
+        __node_base_pointer& __child = __find_leaf_high(
+            __parent, _NodeTypes::__get_key(__src_ptr->__value_));
+        ++__i;
+        __source.__remove_node_pointer(__src_ptr);
+        __insert_node_at(__parent, __child,
+                         static_cast<__node_base_pointer>(__src_ptr));
+    }
+}
+
 #endif  // _LIBCPP_STD_VER > 14
 
 template <class _Tp, class _Compare, class _Allocator>
diff --git a/include/__tuple b/include/__tuple
index 69d6ee961113..3b23d78afa1b 100644
--- a/include/__tuple
+++ b/include/__tuple
@@ -22,36 +22,36 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <class _Tp> class _LIBCPP_TEMPLATE_VIS tuple_size;
+template <class _Tp> struct _LIBCPP_TEMPLATE_VIS tuple_size;
 
 #if !defined(_LIBCPP_CXX03_LANG)
 template <class _Tp, class...>
 using __enable_if_tuple_size_imp = _Tp;
 
 template <class _Tp>
-class _LIBCPP_TEMPLATE_VIS tuple_size<__enable_if_tuple_size_imp<
+struct _LIBCPP_TEMPLATE_VIS tuple_size<__enable_if_tuple_size_imp<
     const _Tp,
     typename enable_if<!is_volatile<_Tp>::value>::type,
     integral_constant<size_t, sizeof(tuple_size<_Tp>)>>>
     : public integral_constant<size_t, tuple_size<_Tp>::value> {};
 
 template <class _Tp>
-class _LIBCPP_TEMPLATE_VIS tuple_size<__enable_if_tuple_size_imp<
+struct _LIBCPP_TEMPLATE_VIS tuple_size<__enable_if_tuple_size_imp<
     volatile _Tp,
     typename enable_if<!is_const<_Tp>::value>::type,
     integral_constant<size_t, sizeof(tuple_size<_Tp>)>>>
     : public integral_constant<size_t, tuple_size<_Tp>::value> {};
 
 template <class _Tp>
-class _LIBCPP_TEMPLATE_VIS tuple_size<__enable_if_tuple_size_imp<
+struct _LIBCPP_TEMPLATE_VIS tuple_size<__enable_if_tuple_size_imp<
     const volatile _Tp,
     integral_constant<size_t, sizeof(tuple_size<_Tp>)>>>
     : public integral_constant<size_t, tuple_size<_Tp>::value> {};
 
 #else
-template <class _Tp> class _LIBCPP_TEMPLATE_VIS tuple_size<const _Tp> : public tuple_size<_Tp> {};
-template <class _Tp> class _LIBCPP_TEMPLATE_VIS tuple_size<volatile _Tp> : public tuple_size<_Tp> {};
-template <class _Tp> class _LIBCPP_TEMPLATE_VIS tuple_size<const volatile _Tp> : public tuple_size<_Tp> {};
+template <class _Tp> struct _LIBCPP_TEMPLATE_VIS tuple_size<const _Tp> : public tuple_size<_Tp> {};
+template <class _Tp> struct _LIBCPP_TEMPLATE_VIS tuple_size<volatile _Tp> : public tuple_size<_Tp> {};
+template <class _Tp> struct _LIBCPP_TEMPLATE_VIS tuple_size<const volatile _Tp> : public tuple_size<_Tp> {};
 #endif
 
 template <size_t _Ip, class _Tp> class _LIBCPP_TEMPLATE_VIS tuple_element;
@@ -165,7 +165,7 @@ template <class ..._Tp> class _LIBCPP_TEMPLATE_VIS tuple;
 template <class... _Tp> struct __tuple_like<tuple<_Tp...> > : true_type {};
 
 template <class ..._Tp>
-class _LIBCPP_TEMPLATE_VIS tuple_size<tuple<_Tp...> >
+struct _LIBCPP_TEMPLATE_VIS tuple_size<tuple<_Tp...> >
     : public integral_constant<size_t, sizeof...(_Tp)>
 {
 };
@@ -291,7 +291,7 @@ public:
 
 
 template <class ..._Tp>
-class _LIBCPP_TEMPLATE_VIS tuple_size<__tuple_types<_Tp...> >
+struct _LIBCPP_TEMPLATE_VIS tuple_size<__tuple_types<_Tp...> >
     : public integral_constant<size_t, sizeof...(_Tp)>
 {
 };
diff --git a/include/algorithm b/include/algorithm
index 90f1d246c63d..d102899f2dfc 100644
--- a/include/algorithm
+++ b/include/algorithm
@@ -645,13 +645,8 @@ template <class BidirectionalIterator, class Compare>
 #include <functional>
 #include <iterator>
 #include <cstddef>
-
-#if defined(__IBMCPP__)
-#include "support/ibm/support.h"
-#endif
-#if defined(_LIBCPP_COMPILER_MSVC)
-#include <intrin.h>
-#endif
+#include <bit>
+#include <version>
 
 #include <__debug>
 
@@ -755,6 +750,32 @@ public:
     bool operator()(const _T1& __x, const _T2& __y) {return __p_(__y, __x);}
 };
 
+// Perform division by two quickly for positive integers (llvm.org/PR39129)
+
+template <typename _Integral>
+_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
+typename enable_if
+<
+    is_integral<_Integral>::value,
+    _Integral
+>::type
+__half_positive(_Integral __value)
+{
+    return static_cast<_Integral>(static_cast<typename make_unsigned<_Integral>::type>(__value) / 2);
+}
+
+template <typename _Tp>
+_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
+typename enable_if
+<
+    !is_integral<_Tp>::value,
+    _Tp
+>::type
+__half_positive(_Tp __value)
+{
+    return __value / 2;
+}
+
 #ifdef _LIBCPP_DEBUG
 
 template <class _Compare>
@@ -788,135 +809,6 @@ struct __debug_less
 
 #endif  // _LIBCPP_DEBUG
 
-// Precondition:  __x != 0
-inline _LIBCPP_INLINE_VISIBILITY
-unsigned __ctz(unsigned __x) {
-#ifndef _LIBCPP_COMPILER_MSVC
-    return static_cast<unsigned>(__builtin_ctz(__x));
-#else
-  static_assert(sizeof(unsigned) == sizeof(unsigned long), "");
-  static_assert(sizeof(unsigned long) == 4, "");
-  unsigned long where;
-  // Search from LSB to MSB for first set bit.
-  // Returns zero if no set bit is found.
-  if (_BitScanForward(&where, __x))
-    return where;
-  return 32;
-#endif
-}
-
-inline _LIBCPP_INLINE_VISIBILITY
-unsigned long __ctz(unsigned long __x) {
-#ifndef _LIBCPP_COMPILER_MSVC
-    return static_cast<unsigned long>(__builtin_ctzl(__x));
-#else
-    static_assert(sizeof(unsigned long) == sizeof(unsigned), "");
-    return __ctz(static_cast<unsigned>(__x));
-#endif
-}
-
-inline _LIBCPP_INLINE_VISIBILITY
-unsigned long long __ctz(unsigned long long __x) {
-#ifndef _LIBCPP_COMPILER_MSVC
-    return static_cast<unsigned long long>(__builtin_ctzll(__x));
-#else
-    unsigned long where;
-// Search from LSB to MSB for first set bit.
-// Returns zero if no set bit is found.
-#if defined(_LIBCPP_HAS_BITSCAN64)
-    (defined(_M_AMD64) || defined(__x86_64__))
-  if (_BitScanForward64(&where, __x))
-    return static_cast<int>(where);
-#else
-  // Win32 doesn't have _BitScanForward64 so emulate it with two 32 bit calls.
-  // Scan the Low Word.
-  if (_BitScanForward(&where, static_cast<unsigned long>(__x)))
-    return where;
-  // Scan the High Word.
-  if (_BitScanForward(&where, static_cast<unsigned long>(__x >> 32)))
-    return where + 32; // Create a bit offset from the LSB.
-#endif
-  return 64;
-#endif // _LIBCPP_COMPILER_MSVC
-}
-
-// Precondition:  __x != 0
-inline _LIBCPP_INLINE_VISIBILITY
-unsigned __clz(unsigned __x) {
-#ifndef _LIBCPP_COMPILER_MSVC
-    return static_cast<unsigned>(__builtin_clz(__x));
-#else
-  static_assert(sizeof(unsigned) == sizeof(unsigned long), "");
-  static_assert(sizeof(unsigned long) == 4, "");
-  unsigned long where;
-  // Search from LSB to MSB for first set bit.
-  // Returns zero if no set bit is found.
-  if (_BitScanReverse(&where, __x))
-    return 31 - where;
-  return 32; // Undefined Behavior.
-#endif
-}
-
-inline _LIBCPP_INLINE_VISIBILITY
-unsigned long __clz(unsigned long __x) {
-#ifndef _LIBCPP_COMPILER_MSVC
-    return static_cast<unsigned long>(__builtin_clzl (__x));
-#else
-    static_assert(sizeof(unsigned) == sizeof(unsigned long), "");
-    return __clz(static_cast<unsigned>(__x));
-#endif
-}
-
-inline _LIBCPP_INLINE_VISIBILITY
-unsigned long long __clz(unsigned long long __x) {
-#ifndef _LIBCPP_COMPILER_MSVC
-    return static_cast<unsigned long long>(__builtin_clzll(__x));
-#else
-  unsigned long where;
-// BitScanReverse scans from MSB to LSB for first set bit.
-// Returns 0 if no set bit is found.
-#if defined(_LIBCPP_HAS_BITSCAN64)
-  if (_BitScanReverse64(&where, __x))
-    return static_cast<int>(63 - where);
-#else
-  // Scan the high 32 bits.
-  if (_BitScanReverse(&where, static_cast<unsigned long>(__x >> 32)))
-    return 63 - (where + 32); // Create a bit offset from the MSB.
-  // Scan the low 32 bits.
-  if (_BitScanReverse(&where, static_cast<unsigned long>(__x)))
-    return 63 - where;
-#endif
-  return 64; // Undefined Behavior.
-#endif // _LIBCPP_COMPILER_MSVC
-}
-
-inline _LIBCPP_INLINE_VISIBILITY int __pop_count(unsigned __x) {
-#ifndef _LIBCPP_COMPILER_MSVC
-  return __builtin_popcount  (__x);
-#else
-  static_assert(sizeof(unsigned) == 4, "");
-  return __popcnt(__x);
-#endif
-}
-
-inline _LIBCPP_INLINE_VISIBILITY int __pop_count(unsigned long __x) {
-#ifndef _LIBCPP_COMPILER_MSVC
-  return __builtin_popcountl (__x);
-#else
-  static_assert(sizeof(unsigned long) == 4, "");
-  return __popcnt(__x);
-#endif
-}
-
-inline _LIBCPP_INLINE_VISIBILITY int __pop_count(unsigned long long __x) {
-#ifndef _LIBCPP_COMPILER_MSVC
-  return __builtin_popcountll(__x);
-#else
-  static_assert(sizeof(unsigned long long) == 8, "");
-  return __popcnt64(__x);
-#endif
-}
-
 // all_of
 
 template <class _InputIterator, class _Predicate>
@@ -2533,6 +2425,8 @@ inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
 _ForwardIterator
 min_element(_ForwardIterator __first, _ForwardIterator __last, _Compare __comp)
 {
+    static_assert(__is_forward_iterator<_ForwardIterator>::value,
+        "std::min_element requires a ForwardIterator");
     if (__first != __last)
     {
         _ForwardIterator __i = __first;
@@ -2597,6 +2491,8 @@ inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
 _ForwardIterator
 max_element(_ForwardIterator __first, _ForwardIterator __last, _Compare __comp)
 {
+    static_assert(__is_forward_iterator<_ForwardIterator>::value,
+        "std::max_element requires a ForwardIterator");
     if (__first != __last)
     {
         _ForwardIterator __i = __first;
@@ -2683,6 +2579,8 @@ _LIBCPP_CONSTEXPR_AFTER_CXX11
 std::pair<_ForwardIterator, _ForwardIterator>
 minmax_element(_ForwardIterator __first, _ForwardIterator __last, _Compare __comp)
 {
+  static_assert(__is_forward_iterator<_ForwardIterator>::value,
+        "std::minmax_element requires a ForwardIterator");
   std::pair<_ForwardIterator, _ForwardIterator> __result(__first, __first);
   if (__first != __last)
   {
@@ -3027,10 +2925,11 @@ template<class _IntType>
 template<class _URNG>
 typename uniform_int_distribution<_IntType>::result_type
 uniform_int_distribution<_IntType>::operator()(_URNG& __g, const param_type& __p)
+_LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK
 {
     typedef typename conditional<sizeof(result_type) <= sizeof(uint32_t),
                                             uint32_t, uint64_t>::type _UIntType;
-    const _UIntType _Rp = __p.b() - __p.a() + _UIntType(1);
+    const _UIntType _Rp = _UIntType(__p.b()) - _UIntType(__p.a()) + _UIntType(1);
     if (_Rp == 1)
         return __p.a();
     const size_t _Dt = numeric_limits<_UIntType>::digits;
@@ -3080,7 +2979,7 @@ public:
 _LIBCPP_FUNC_VIS __rs_default __rs_get();
 
 template <class _RandomAccessIterator>
-void
+_LIBCPP_DEPRECATED_IN_CXX14 void
 random_shuffle(_RandomAccessIterator __first, _RandomAccessIterator __last)
 {
     typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;
@@ -3101,7 +3000,7 @@ random_shuffle(_RandomAccessIterator __first, _RandomAccessIterator __last)
 }
 
 template <class _RandomAccessIterator, class _RandomNumberGenerator>
-void
+_LIBCPP_DEPRECATED_IN_CXX14 void
 random_shuffle(_RandomAccessIterator __first, _RandomAccessIterator __last,
 #ifndef _LIBCPP_CXX03_LANG
                _RandomNumberGenerator&& __rand)
@@ -3116,7 +3015,8 @@ random_shuffle(_RandomAccessIterator __first, _RandomAccessIterator __last,
         for (--__last; __first < __last; ++__first, --__d)
         {
             difference_type __i = __rand(__d);
-            swap(*__first, *(__first + __i));
+            if (__i != difference_type(0))
+              swap(*__first, *(__first + __i));
         }
     }
 }
@@ -3328,7 +3228,7 @@ partition_point(_ForwardIterator __first, _ForwardIterator __last, _Predicate __
     difference_type __len = _VSTD::distance(__first, __last);
     while (__len != 0)
     {
-        difference_type __l2 = __len / 2;
+        difference_type __l2 = _VSTD::__half_positive(__len);
         _ForwardIterator __m = __first;
         _VSTD::advance(__m, __l2);
         if (__pred(*__m))
@@ -3737,6 +3637,7 @@ __sort4(_ForwardIterator __x1, _ForwardIterator __x2, _ForwardIterator __x3,
 // stable, 4-10 compares, 0-9 swaps
 
 template <class _Compare, class _ForwardIterator>
+_LIBCPP_HIDDEN
 unsigned
 __sort5(_ForwardIterator __x1, _ForwardIterator __x2, _ForwardIterator __x3,
             _ForwardIterator __x4, _ForwardIterator __x5, _Compare __c)
@@ -4195,7 +4096,7 @@ __lower_bound(_ForwardIterator __first, _ForwardIterator __last, const _Tp& __va
     difference_type __len = _VSTD::distance(__first, __last);
     while (__len != 0)
     {
-        difference_type __l2 = __len / 2;
+        difference_type __l2 = _VSTD::__half_positive(__len);
         _ForwardIterator __m = __first;
         _VSTD::advance(__m, __l2);
         if (__comp(*__m, __value_))
@@ -4214,14 +4115,8 @@ inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 _ForwardIterator
 lower_bound(_ForwardIterator __first, _ForwardIterator __last, const _Tp& __value_, _Compare __comp)
 {
-#ifdef _LIBCPP_DEBUG
-    typedef typename add_lvalue_reference<__debug_less<_Compare> >::type _Comp_ref;
-    __debug_less<_Compare> __c(__comp);
-    return __lower_bound<_Comp_ref>(__first, __last, __value_, __c);
-#else  // _LIBCPP_DEBUG
     typedef typename add_lvalue_reference<_Compare>::type _Comp_ref;
     return __lower_bound<_Comp_ref>(__first, __last, __value_, __comp);
-#endif  // _LIBCPP_DEBUG
 }
 
 template <class _ForwardIterator, class _Tp>
@@ -4243,7 +4138,7 @@ __upper_bound(_ForwardIterator __first, _ForwardIterator __last, const _Tp& __va
     difference_type __len = _VSTD::distance(__first, __last);
     while (__len != 0)
     {
-        difference_type __l2 = __len / 2;
+        difference_type __l2 = _VSTD::__half_positive(__len);
         _ForwardIterator __m = __first;
         _VSTD::advance(__m, __l2);
         if (__comp(__value_, *__m))
@@ -4262,14 +4157,8 @@ inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 _ForwardIterator
 upper_bound(_ForwardIterator __first, _ForwardIterator __last, const _Tp& __value_, _Compare __comp)
 {
-#ifdef _LIBCPP_DEBUG
-    typedef typename add_lvalue_reference<__debug_less<_Compare> >::type _Comp_ref;
-    __debug_less<_Compare> __c(__comp);
-    return __upper_bound<_Comp_ref>(__first, __last, __value_, __c);
-#else  // _LIBCPP_DEBUG
     typedef typename add_lvalue_reference<_Compare>::type _Comp_ref;
     return __upper_bound<_Comp_ref>(__first, __last, __value_, __comp);
-#endif  // _LIBCPP_DEBUG
 }
 
 template <class _ForwardIterator, class _Tp>
@@ -4291,7 +4180,7 @@ __equal_range(_ForwardIterator __first, _ForwardIterator __last, const _Tp& __va
     difference_type __len = _VSTD::distance(__first, __last);
     while (__len != 0)
     {
-        difference_type __l2 = __len / 2;
+        difference_type __l2 = _VSTD::__half_positive(__len);
         _ForwardIterator __m = __first;
         _VSTD::advance(__m, __l2);
         if (__comp(*__m, __value_))
diff --git a/include/any b/include/any
index 9bd2f53c5601..781eee7869c0 100644
--- a/include/any
+++ b/include/any
@@ -87,13 +87,14 @@ namespace std {
 #include <typeinfo>
 #include <type_traits>
 #include <cstdlib>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 namespace std {
-class _LIBCPP_EXCEPTION_ABI bad_any_cast : public bad_cast
+class _LIBCPP_EXCEPTION_ABI _LIBCPP_AVAILABILITY_BAD_ANY_CAST bad_any_cast : public bad_cast
 {
 public:
     virtual const char* what() const _NOEXCEPT;
@@ -105,12 +106,13 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 #if _LIBCPP_STD_VER > 14
 
 _LIBCPP_NORETURN inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_AVAILABILITY_THROW_BAD_ANY_CAST
 void __throw_bad_any_cast()
 {
 #ifndef _LIBCPP_NO_EXCEPTIONS
     throw bad_any_cast();
 #else
-	_VSTD::abort();
+    _VSTD::abort();
 #endif
 }
 
@@ -576,6 +578,7 @@ any make_any(initializer_list<_Up> __il, _Args&&... __args) {
 
 template <class _ValueType>
 inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_AVAILABILITY_THROW_BAD_ANY_CAST
 _ValueType any_cast(any const & __v)
 {
     using _RawValueType = __uncvref_t<_ValueType>;
@@ -590,6 +593,7 @@ _ValueType any_cast(any const & __v)
 
 template <class _ValueType>
 inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_AVAILABILITY_THROW_BAD_ANY_CAST
 _ValueType any_cast(any & __v)
 {
     using _RawValueType = __uncvref_t<_ValueType>;
@@ -604,6 +608,7 @@ _ValueType any_cast(any & __v)
 
 template <class _ValueType>
 inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_AVAILABILITY_THROW_BAD_ANY_CAST
 _ValueType any_cast(any && __v)
 {
     using _RawValueType = __uncvref_t<_ValueType>;
diff --git a/include/array b/include/array
index 1e6a650198b5..56f6887655aa 100644
--- a/include/array
+++ b/include/array
@@ -91,7 +91,7 @@ template <class T, size_t N>
 template <class T, size_t N >
   void swap(array<T,N>& x, array<T,N>& y) noexcept(noexcept(x.swap(y))); // C++17
 
-template <class T> class tuple_size;
+template <class T> struct tuple_size;
 template <size_t I, class T> class tuple_element;
 template <class T, size_t N> struct tuple_size<array<T, N>>;
 template <size_t I, class T, size_t N> struct tuple_element<I, array<T, N>>;
@@ -112,6 +112,7 @@ template <size_t I, class T, size_t N> const T&& get(const array<T, N>&&) noexce
 #include <algorithm>
 #include <stdexcept>
 #include <cstdlib> // for _LIBCPP_UNREACHABLE
+#include <version>
 #include <__debug>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -367,7 +368,7 @@ array(_Tp, _Args...)
 
 template <class _Tp, size_t _Size>
 inline _LIBCPP_INLINE_VISIBILITY
-bool
+_LIBCPP_CONSTEXPR_AFTER_CXX17 bool
 operator==(const array<_Tp, _Size>& __x, const array<_Tp, _Size>& __y)
 {
     return _VSTD::equal(__x.begin(), __x.end(), __y.begin());
@@ -375,7 +376,7 @@ operator==(const array<_Tp, _Size>& __x, const array<_Tp, _Size>& __y)
 
 template <class _Tp, size_t _Size>
 inline _LIBCPP_INLINE_VISIBILITY
-bool
+_LIBCPP_CONSTEXPR_AFTER_CXX17 bool
 operator!=(const array<_Tp, _Size>& __x, const array<_Tp, _Size>& __y)
 {
     return !(__x == __y);
@@ -383,7 +384,7 @@ operator!=(const array<_Tp, _Size>& __x, const array<_Tp, _Size>& __y)
 
 template <class _Tp, size_t _Size>
 inline _LIBCPP_INLINE_VISIBILITY
-bool
+_LIBCPP_CONSTEXPR_AFTER_CXX17 bool
 operator<(const array<_Tp, _Size>& __x, const array<_Tp, _Size>& __y)
 {
     return _VSTD::lexicographical_compare(__x.begin(), __x.end(),
@@ -392,7 +393,7 @@ operator<(const array<_Tp, _Size>& __x, const array<_Tp, _Size>& __y)
 
 template <class _Tp, size_t _Size>
 inline _LIBCPP_INLINE_VISIBILITY
-bool
+_LIBCPP_CONSTEXPR_AFTER_CXX17 bool
 operator>(const array<_Tp, _Size>& __x, const array<_Tp, _Size>& __y)
 {
     return __y < __x;
@@ -400,7 +401,7 @@ operator>(const array<_Tp, _Size>& __x, const array<_Tp, _Size>& __y)
 
 template <class _Tp, size_t _Size>
 inline _LIBCPP_INLINE_VISIBILITY
-bool
+_LIBCPP_CONSTEXPR_AFTER_CXX17 bool
 operator<=(const array<_Tp, _Size>& __x, const array<_Tp, _Size>& __y)
 {
     return !(__y < __x);
@@ -408,7 +409,7 @@ operator<=(const array<_Tp, _Size>& __x, const array<_Tp, _Size>& __y)
 
 template <class _Tp, size_t _Size>
 inline _LIBCPP_INLINE_VISIBILITY
-bool
+_LIBCPP_CONSTEXPR_AFTER_CXX17 bool
 operator>=(const array<_Tp, _Size>& __x, const array<_Tp, _Size>& __y)
 {
     return !(__x < __y);
@@ -429,7 +430,7 @@ swap(array<_Tp, _Size>& __x, array<_Tp, _Size>& __y)
 }
 
 template <class _Tp, size_t _Size>
-class _LIBCPP_TEMPLATE_VIS tuple_size<array<_Tp, _Size> >
+struct _LIBCPP_TEMPLATE_VIS tuple_size<array<_Tp, _Size> >
     : public integral_constant<size_t, _Size> {};
 
 template <size_t _Ip, class _Tp, size_t _Size>
diff --git a/include/atomic b/include/atomic
index 809f78a06d36..d37e7b4b035c 100644
--- a/include/atomic
+++ b/include/atomic
@@ -544,6 +544,7 @@ void atomic_signal_fence(memory_order m) noexcept;
 #include <cstddef>
 #include <cstdint>
 #include <type_traits>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
@@ -559,10 +560,6 @@ void atomic_signal_fence(memory_order m) noexcept;
 #error C++ standard library is incompatible with <stdatomic.h>
 #endif
 
-#if _LIBCPP_STD_VER > 14
-# define __cpp_lib_atomic_is_always_lock_free 201603L
-#endif
-
 #define _LIBCPP_CHECK_STORE_MEMORY_ORDER(__m) \
   _LIBCPP_DIAGNOSE_WARNING(__m == memory_order_consume || \
                            __m == memory_order_acquire || \
diff --git a/include/bit b/include/bit
new file mode 100644
index 000000000000..db3812e5b5b2
--- /dev/null
+++ b/include/bit
@@ -0,0 +1,158 @@
+// -*- C++ -*-
+//===------------------------------ bit ----------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===---------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_BIT
+#define _LIBCPP_BIT
+
+/*
+    bit synopsis
+
+namespace std {
+
+} // namespace std
+
+*/
+
+#include <__config>
+#include <version>
+
+#if defined(__IBMCPP__)
+#include "support/ibm/support.h"
+#endif
+#if defined(_LIBCPP_COMPILER_MSVC)
+#include <intrin.h>
+#endif
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#ifndef _LIBCPP_COMPILER_MSVC
+
+inline _LIBCPP_INLINE_VISIBILITY
+int __ctz(unsigned __x)           { return __builtin_ctz(__x); }
+
+inline _LIBCPP_INLINE_VISIBILITY
+int __ctz(unsigned long __x)      { return __builtin_ctzl(__x); }
+
+inline _LIBCPP_INLINE_VISIBILITY
+int __ctz(unsigned long long __x) { return __builtin_ctzll(__x); }
+
+
+inline _LIBCPP_INLINE_VISIBILITY
+int __clz(unsigned __x)           { return __builtin_clz(__x); }
+
+inline _LIBCPP_INLINE_VISIBILITY
+int __clz(unsigned long __x)      { return __builtin_clzl(__x); }
+
+inline _LIBCPP_INLINE_VISIBILITY
+int __clz(unsigned long long __x) { return __builtin_clzll(__x); }
+
+
+inline _LIBCPP_INLINE_VISIBILITY
+int __popcount(unsigned __x)           { return __builtin_popcount(__x); }
+
+inline _LIBCPP_INLINE_VISIBILITY
+int __popcount(unsigned long __x)      { return __builtin_popcountl(__x); }
+
+inline _LIBCPP_INLINE_VISIBILITY
+int __popcount(unsigned long long __x) { return __builtin_popcountll(__x); }
+
+#else  // _LIBCPP_COMPILER_MSVC
+
+// Precondition:  __x != 0
+inline _LIBCPP_INLINE_VISIBILITY
+int __ctz(unsigned __x) {
+  static_assert(sizeof(unsigned) == sizeof(unsigned long), "");
+  static_assert(sizeof(unsigned long) == 4, "");
+  unsigned long __where;
+  if (_BitScanForward(&__where, __x))
+    return static_cast<int>(__where);
+  return 32;
+}
+
+inline _LIBCPP_INLINE_VISIBILITY
+int __ctz(unsigned long __x) {
+    static_assert(sizeof(unsigned long) == sizeof(unsigned), "");
+    return __ctz(static_cast<unsigned>(__x));
+}
+
+inline _LIBCPP_INLINE_VISIBILITY
+int __ctz(unsigned long long __x) {
+    unsigned long __where;
+#if defined(_LIBCPP_HAS_BITSCAN64)
+    (defined(_M_AMD64) || defined(__x86_64__))
+  if (_BitScanForward64(&__where, __x))
+    return static_cast<int>(__where);
+#else
+  // Win32 doesn't have _BitScanForward64 so emulate it with two 32 bit calls.
+  if (_BitScanForward(&__where, static_cast<unsigned long>(__x)))
+    return static_cast<int>(__where);
+  if (_BitScanForward(&__where, static_cast<unsigned long>(__x >> 32)))
+    return static_cast<int>(__where + 32);
+#endif
+  return 64;
+}
+
+// Precondition:  __x != 0
+inline _LIBCPP_INLINE_VISIBILITY
+int __clz(unsigned __x) {
+  static_assert(sizeof(unsigned) == sizeof(unsigned long), "");
+  static_assert(sizeof(unsigned long) == 4, "");
+  unsigned long __where;
+  if (_BitScanReverse(&__where, __x))
+    return static_cast<int>(31 - __where);
+  return 32; // Undefined Behavior.
+}
+
+inline _LIBCPP_INLINE_VISIBILITY
+int __clz(unsigned long __x) {
+    static_assert(sizeof(unsigned) == sizeof(unsigned long), "");
+    return __clz(static_cast<unsigned>(__x));
+}
+
+inline _LIBCPP_INLINE_VISIBILITY
+int __clz(unsigned long long __x) {
+  unsigned long __where;
+#if defined(_LIBCPP_HAS_BITSCAN64)
+  if (_BitScanReverse64(&__where, __x))
+    return static_cast<int>(63 - __where);
+#else
+  // Win32 doesn't have _BitScanReverse64 so emulate it with two 32 bit calls.
+  if (_BitScanReverse(&__where, static_cast<unsigned long>(__x >> 32)))
+    return static_cast<int>(63 - (__where + 32));
+  if (_BitScanReverse(&__where, static_cast<unsigned long>(__x)))
+    return static_cast<int>(63 - __where);
+#endif
+  return 64; // Undefined Behavior.
+}
+
+inline _LIBCPP_INLINE_VISIBILITY int __popcount(unsigned __x) {
+  static_assert(sizeof(unsigned) == 4, "");
+  return __popcnt(__x);
+}
+
+inline _LIBCPP_INLINE_VISIBILITY int __popcount(unsigned long __x) {
+  static_assert(sizeof(unsigned long) == 4, "");
+  return __popcnt(__x);
+}
+
+inline _LIBCPP_INLINE_VISIBILITY int __popcount(unsigned long long __x) {
+  static_assert(sizeof(unsigned long long) == 8, "");
+  return __popcnt64(__x);
+}
+
+#endif // _LIBCPP_COMPILER_MSVC
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_BIT
diff --git a/include/bitset b/include/bitset
index cd6c289b4c48..98947e027c1c 100644
--- a/include/bitset
+++ b/include/bitset
@@ -238,9 +238,9 @@ __bitset<_N_words, _Size>::__init(unsigned long long __v, false_type) _NOEXCEPT
     size_t __sz = _Size;
     for (size_t __i = 0; __i < sizeof(__t)/sizeof(__t[0]); ++__i, __v >>= __bits_per_word, __sz -= __bits_per_word )
         if ( __sz < __bits_per_word)
-        	__t[__i] = static_cast<__storage_type>(__v) & ( 1ULL << __sz ) - 1;
+            __t[__i] = static_cast<__storage_type>(__v) & ( 1ULL << __sz ) - 1;
         else
-        	__t[__i] = static_cast<__storage_type>(__v);
+            __t[__i] = static_cast<__storage_type>(__v);
 
     _VSTD::copy(__t, __t + sizeof(__t)/sizeof(__t[0]), __first_);
     _VSTD::fill(__first_ + sizeof(__t)/sizeof(__t[0]), __first_ + sizeof(__first_)/sizeof(__first_[0]),
@@ -254,7 +254,7 @@ __bitset<_N_words, _Size>::__init(unsigned long long __v, true_type) _NOEXCEPT
 {
     __first_[0] = __v;
     if (_Size < __bits_per_word)
-    	__first_[0] &= ( 1ULL << _Size ) - 1;
+        __first_[0] &= ( 1ULL << _Size ) - 1;
 
     _VSTD::fill(__first_ + 1, __first_ + sizeof(__first_)/sizeof(__first_[0]), __storage_type(0));
 }
@@ -269,9 +269,9 @@ __bitset<_N_words, _Size>::__bitset(unsigned long long __v) _NOEXCEPT
 #if __SIZEOF_SIZE_T__ == 8
     : __first_{__v}
 #elif __SIZEOF_SIZE_T__ == 4
-    : __first_{static_cast<__storage_type>(__v), 
-    			_Size >= 2 * __bits_per_word ? static_cast<__storage_type>(__v >> __bits_per_word)
-    			: static_cast<__storage_type>((__v >> __bits_per_word) & (__storage_type(1) << (_Size - __bits_per_word)) - 1)}
+    : __first_{static_cast<__storage_type>(__v),
+                _Size >= 2 * __bits_per_word ? static_cast<__storage_type>(__v >> __bits_per_word)
+                : static_cast<__storage_type>((__v >> __bits_per_word) & (__storage_type(1) << (_Size - __bits_per_word)) - 1)}
 #else
 #error This constructor has not been ported to this platform
 #endif
@@ -991,7 +991,7 @@ inline
 size_t
 bitset<_Size>::count() const _NOEXCEPT
 {
-    return static_cast<size_t>(_VSTD::count(base::__make_iter(0), base::__make_iter(_Size), true));
+    return static_cast<size_t>(__count_bool_true(base::__make_iter(0), _Size));
 }
 
 template <size_t _Size>
diff --git a/include/charconv b/include/charconv
index 7cb790e1beec..064f2e11c3f3 100644
--- a/include/charconv
+++ b/include/charconv
@@ -87,8 +87,16 @@ namespace std {
 #pragma GCC system_header
 #endif
 
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+namespace __itoa {
+_LIBCPP_FUNC_VIS char* __u64toa(uint64_t __value, char* __buffer);
+_LIBCPP_FUNC_VIS char* __u32toa(uint32_t __value, char* __buffer);
+}
+
 #if _LIBCPP_STD_VER > 11
 
 enum class _LIBCPP_ENUM_VIS chars_format
@@ -147,9 +155,6 @@ static constexpr uint32_t __pow10_32[] = {
     UINT32_C(1000000000),
 };
 
-_LIBCPP_FUNC_VIS char* __u64toa(uint64_t __value, char* __buffer);
-_LIBCPP_FUNC_VIS char* __u32toa(uint32_t __value, char* __buffer);
-
 template <typename _Tp, typename = void>
 struct _LIBCPP_HIDDEN __traits_base
 {
@@ -607,4 +612,6 @@ from_chars(const char* __first, const char* __last, _Tp& __value, int __base)
 
 _LIBCPP_END_NAMESPACE_STD
 
+_LIBCPP_POP_MACROS
+
 #endif  // _LIBCPP_CHARCONV
diff --git a/include/chrono b/include/chrono
index f6a6f4b24343..96759f9860ee 100644
--- a/include/chrono
+++ b/include/chrono
@@ -33,9 +33,9 @@ template <class Rep>
 struct duration_values
 {
 public:
-    static constexpr Rep zero();
-    static constexpr Rep max();
-    static constexpr Rep min();
+    static constexpr Rep zero(); // noexcept in C++20
+    static constexpr Rep max();  // noexcept in C++20
+    static constexpr Rep min();  // noexcept in C++20
 };
 
 // duration
@@ -77,22 +77,24 @@ public:
 
     constexpr common_type<duration>::type  operator+() const;
     constexpr common_type<duration>::type  operator-() const;
-    constexpr duration& operator++();
-    constexpr duration  operator++(int);
-    constexpr duration& operator--();
-    constexpr duration  operator--(int);
+    constexpr duration& operator++();    // constexpr in C++17
+    constexpr duration  operator++(int); // constexpr in C++17
+    constexpr duration& operator--();    // constexpr in C++17
+    constexpr duration  operator--(int); // constexpr in C++17
 
-    constexpr duration& operator+=(const duration& d);
-    constexpr duration& operator-=(const duration& d);
+    constexpr duration& operator+=(const duration& d);  // constexpr in C++17
+    constexpr duration& operator-=(const duration& d);  // constexpr in C++17
 
-    duration& operator*=(const rep& rhs);
-    duration& operator/=(const rep& rhs);
+    duration& operator*=(const rep& rhs);       // constexpr in C++17
+    duration& operator/=(const rep& rhs);       // constexpr in C++17
+    duration& operator%=(const rep& rhs);       // constexpr in C++17
+    duration& operator%=(const duration& rhs);  // constexpr in C++17
 
     // special values
 
-    static constexpr duration zero();
-    static constexpr duration min();
-    static constexpr duration max();
+    static constexpr duration zero(); // noexcept in C++20
+    static constexpr duration min();  // noexcept in C++20
+    static constexpr duration max();  // noexcept in C++20
 };
 
 typedef duration<long long,         nano> nanoseconds;
@@ -127,13 +129,13 @@ public:
 
     // arithmetic
 
-    time_point& operator+=(const duration& d);
-    time_point& operator-=(const duration& d);
+    time_point& operator+=(const duration& d); // constexpr in C++17
+    time_point& operator-=(const duration& d); // constexpr in C++17
 
     // special values
 
-    static constexpr time_point min();
-    static constexpr time_point max();
+    static constexpr time_point min();  // noexcept in C++20
+    static constexpr time_point max();  // noexcept in C++20
 };
 
 } // chrono
@@ -323,7 +325,7 @@ struct clock_time_conversion;
 
 template<class DestClock, class SourceClock, class Duration>
   auto clock_cast(const time_point<SourceClock, Duration>& t);
-  
+
 // 25.8.2, class last_spec    // C++20
 struct last_spec;
 
@@ -528,7 +530,7 @@ constexpr year_month_weekday_last
   operator-(const year_month_weekday_last& ymwdl, const months& dm) noexcept;
 constexpr year_month_weekday_last
   operator-(const year_month_weekday_last& ymwdl, const years& dy) noexcept;
-  
+
 // 25.8.18, civil calendar conventional syntax operators    // C++20
 constexpr year_month
   operator/(const year& y, const month& m) noexcept;
@@ -609,7 +611,7 @@ constexpr year_month_weekday_last
 constexpr year_month_weekday_last
   operator/(const month_weekday_last& mwdl, const year& y) noexcept;
 constexpr year_month_weekday_last
-  operator/(const month_weekday_last& mwdl, int y) noexcept;  
+  operator/(const month_weekday_last& mwdl, int y) noexcept;
 
 // 25.9, class template time_of_day    // C++20
 template<class Duration> class time_of_day;
@@ -640,7 +642,7 @@ class ambiguous_local_time;
 // 25.10.4, information classes    // C++20
 struct sys_info;
 struct local_info;
-    
+
 // 25.10.5, class time_zone    // C++20
 enum class choose {earliest, latest};
 class time_zone;
@@ -650,7 +652,7 @@ bool operator<(const time_zone& x, const time_zone& y) noexcept;
 bool operator>(const time_zone& x, const time_zone& y) noexcept;
 bool operator<=(const time_zone& x, const time_zone& y) noexcept;
 bool operator>=(const time_zone& x, const time_zone& y) noexcept;
-  
+
 // 25.10.6, class template zoned_traits    // C++20
 template<class T> struct zoned_traits;
 
@@ -724,7 +726,7 @@ template<class charT, class traits, class Alloc, class Streamable>
 template<class charT, class traits, class Alloc, class Streamable>
   basic_string<charT, traits, Alloc>
     format(const locale& loc, const basic_string<charT, traits, Alloc>& fmt,
-           const Streamable& s);             
+           const Streamable& s);
 
 // 25.12, parsing    // C++20
 template<class charT, class traits, class Alloc, class Parsable>
@@ -746,7 +748,8 @@ unspecified
     parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp,
           basic_string<charT, traits, Alloc>& abbrev, minutes& offset);
 
-inline constexpr last_spec								last{};       // C++20
+// calendrical constants
+inline constexpr last_spec                              last{};       // C++20
 inline constexpr chrono::weekday                        Sunday{0};    // C++20
 inline constexpr chrono::weekday                        Monday{1};    // C++20
 inline constexpr chrono::weekday                        Tuesday{2};   // C++20
@@ -796,6 +799,7 @@ constexpr chrono::year                                  operator ""y(unsigned lo
 #include <type_traits>
 #include <ratio>
 #include <limits>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
@@ -804,6 +808,11 @@ constexpr chrono::year                                  operator ""y(unsigned lo
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
+#ifndef _LIBCPP_CXX03_LANG
+_LIBCPP_BEGIN_NAMESPACE_FILESYSTEM
+struct _FilesystemClock;
+_LIBCPP_END_NAMESPACE_FILESYSTEM
+#endif // !_LIBCPP_CXX03_LANG
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
@@ -920,9 +929,9 @@ template <class _Rep>
 struct _LIBCPP_TEMPLATE_VIS duration_values
 {
 public:
-    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR _Rep zero() {return _Rep(0);}
-    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR _Rep max()  {return numeric_limits<_Rep>::max();}
-    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR _Rep min()  {return numeric_limits<_Rep>::lowest();}
+    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR _Rep zero() _NOEXCEPT {return _Rep(0);}
+    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR _Rep max()  _NOEXCEPT {return numeric_limits<_Rep>::max();}
+    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR _Rep min()  _NOEXCEPT {return numeric_limits<_Rep>::lowest();}
 };
 
 #if _LIBCPP_STD_VER > 14
@@ -1015,7 +1024,7 @@ class _LIBCPP_TEMPLATE_VIS duration
         typedef ratio<__mul<__n1, __d2, !value>::value,
                       __mul<__n2, __d1, !value>::value> type;
     };
-    
+
 public:
     typedef _Rep rep;
     typedef typename _Period::type period;
@@ -1077,9 +1086,9 @@ public:
 
     // special values
 
-    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR duration zero() {return duration(duration_values<rep>::zero());}
-    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR duration min()  {return duration(duration_values<rep>::min());}
-    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR duration max()  {return duration(duration_values<rep>::max());}
+    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR duration zero() _NOEXCEPT {return duration(duration_values<rep>::zero());}
+    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR duration min()  _NOEXCEPT {return duration(duration_values<rep>::min());}
+    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR duration max()  _NOEXCEPT {return duration(duration_values<rep>::max());}
 };
 
 typedef duration<long long,         nano> nanoseconds;
@@ -1088,7 +1097,12 @@ typedef duration<long long,        milli> milliseconds;
 typedef duration<long long              > seconds;
 typedef duration<     long, ratio<  60> > minutes;
 typedef duration<     long, ratio<3600> > hours;
-
+#if _LIBCPP_STD_VER > 17
+typedef duration<     int, ratio_multiply<ratio<24>, hours::period>>         days;
+typedef duration<     int, ratio_multiply<ratio<7>,   days::period>>         weeks;
+typedef duration<     int, ratio_multiply<ratio<146097, 400>, days::period>> years;
+typedef duration<     int, ratio_divide<years::period, ratio<12>>>           months;
+#endif
 // Duration ==
 
 template <class _LhsDuration, class _RhsDuration>
@@ -1355,13 +1369,13 @@ public:
 
     // arithmetic
 
-    _LIBCPP_INLINE_VISIBILITY time_point& operator+=(const duration& __d) {__d_ += __d; return *this;}
-    _LIBCPP_INLINE_VISIBILITY time_point& operator-=(const duration& __d) {__d_ -= __d; return *this;}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14 time_point& operator+=(const duration& __d) {__d_ += __d; return *this;}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14 time_point& operator-=(const duration& __d) {__d_ -= __d; return *this;}
 
     // special values
 
-    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR time_point min() {return time_point(duration::min());}
-    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR time_point max() {return time_point(duration::max());}
+    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR time_point min() _NOEXCEPT {return time_point(duration::min());}
+    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR time_point max() _NOEXCEPT {return time_point(duration::max());}
 };
 
 } // chrono
@@ -1571,12 +1585,1158 @@ typedef steady_clock high_resolution_clock;
 typedef system_clock high_resolution_clock;
 #endif
 
+#if _LIBCPP_STD_VER > 17
+// [time.clock.file], type file_clock
+using file_clock = _VSTD_FS::_FilesystemClock;
+
+template<class _Duration>
+using file_time = time_point<file_clock, _Duration>;
+
+
+template <class _Duration>
+using sys_time    = time_point<system_clock, _Duration>;
+using sys_seconds = sys_time<seconds>;
+using sys_days    = sys_time<days>;
+
+struct local_t {};
+template<class Duration>
+using local_time  = time_point<local_t, Duration>;
+using local_seconds = local_time<seconds>;
+using local_days    = local_time<days>;
+
+
+struct _LIBCPP_TYPE_VIS last_spec { explicit last_spec() = default; };
+
+class _LIBCPP_TYPE_VIS day {
+private:
+    unsigned char __d;
+public:
+    day() = default;
+    explicit inline constexpr day(unsigned __val) noexcept : __d(static_cast<unsigned char>(__val)) {}
+    inline constexpr day& operator++()    noexcept { ++__d; return *this; }
+    inline constexpr day  operator++(int) noexcept { day __tmp = *this; ++(*this); return __tmp; }
+    inline constexpr day& operator--()    noexcept { --__d; return *this; }
+    inline constexpr day  operator--(int) noexcept { day __tmp = *this; --(*this); return __tmp; }
+           constexpr day& operator+=(const days& __dd) noexcept;
+           constexpr day& operator-=(const days& __dd) noexcept;
+    explicit inline constexpr operator unsigned() const noexcept { return __d; }
+    inline constexpr bool ok() const noexcept { return __d >= 1 && __d <= 31; }
+  };
+
+
+inline constexpr
+bool operator==(const day& __lhs, const day& __rhs) noexcept
+{ return static_cast<unsigned>(__lhs) == static_cast<unsigned>(__rhs); }
+
+inline constexpr
+bool operator!=(const day& __lhs, const day& __rhs) noexcept
+{ return !(__lhs == __rhs); }
+
+inline constexpr
+bool operator< (const day& __lhs, const day& __rhs) noexcept
+{ return static_cast<unsigned>(__lhs) <  static_cast<unsigned>(__rhs); }
+
+inline constexpr
+bool operator> (const day& __lhs, const day& __rhs) noexcept
+{ return __rhs < __lhs; }
+
+inline constexpr
+bool operator<=(const day& __lhs, const day& __rhs) noexcept
+{ return !(__rhs < __lhs);}
+
+inline constexpr
+bool operator>=(const day& __lhs, const day& __rhs) noexcept
+{ return !(__lhs < __rhs); }
+
+inline constexpr
+day operator+ (const day& __lhs, const days& __rhs) noexcept
+{ return day(static_cast<unsigned>(__lhs) + __rhs.count()); }
+
+inline constexpr
+day operator+ (const days& __lhs, const day& __rhs) noexcept
+{ return __rhs + __lhs; }
+
+inline constexpr
+day operator- (const day& __lhs, const days& __rhs) noexcept
+{ return __lhs + -__rhs; }
+
+inline constexpr
+days operator-(const day& __lhs, const day& __rhs) noexcept
+{ return days(static_cast<int>(static_cast<unsigned>(__lhs)) -
+              static_cast<int>(static_cast<unsigned>(__rhs))); }
+
+inline constexpr day& day::operator+=(const days& __dd) noexcept
+{ *this = *this + __dd; return *this; }
+
+inline constexpr day& day::operator-=(const days& __dd) noexcept
+{ *this = *this - __dd; return *this; }
+
+
+class _LIBCPP_TYPE_VIS month {
+private:
+    unsigned char __m;
+public:
+    month() = default;
+    explicit inline constexpr month(unsigned __val) noexcept : __m(static_cast<unsigned char>(__val)) {}
+    inline constexpr month& operator++()    noexcept { ++__m; return *this; }
+    inline constexpr month  operator++(int) noexcept { month __tmp = *this; ++(*this); return __tmp; }
+    inline constexpr month& operator--()    noexcept { --__m; return *this; }
+    inline constexpr month  operator--(int) noexcept { month __tmp = *this; --(*this); return __tmp; }
+           constexpr month& operator+=(const months& __m1) noexcept;
+           constexpr month& operator-=(const months& __m1) noexcept;
+    explicit inline constexpr operator unsigned() const noexcept { return __m; }
+    inline constexpr bool ok() const noexcept { return __m >= 1 && __m <= 12; }
+};
+
+
+inline constexpr
+bool operator==(const month& __lhs, const month& __rhs) noexcept
+{ return static_cast<unsigned>(__lhs) == static_cast<unsigned>(__rhs); }
+
+inline constexpr
+bool operator!=(const month& __lhs, const month& __rhs) noexcept
+{ return !(__lhs == __rhs); }
+
+inline constexpr
+bool operator< (const month& __lhs, const month& __rhs) noexcept
+{ return static_cast<unsigned>(__lhs)  < static_cast<unsigned>(__rhs); }
+
+inline constexpr
+bool operator> (const month& __lhs, const month& __rhs) noexcept
+{ return __rhs < __lhs; }
+
+inline constexpr
+bool operator<=(const month& __lhs, const month& __rhs) noexcept
+{ return !(__rhs < __lhs); }
+
+inline constexpr
+bool operator>=(const month& __lhs, const month& __rhs) noexcept
+{ return !(__lhs < __rhs); }
+
+inline constexpr
+month operator+ (const month& __lhs, const months& __rhs) noexcept
+{
+    auto const __mu = static_cast<long long>(static_cast<unsigned>(__lhs)) + (__rhs.count() - 1);
+    auto const __yr = (__mu >= 0 ? __mu : __mu - 11) / 12;
+    return month{static_cast<unsigned>(__mu - __yr * 12 + 1)};
+}
+
+inline constexpr
+month operator+ (const months& __lhs, const month& __rhs) noexcept
+{ return __rhs + __lhs; }
+
+inline constexpr
+month operator- (const month& __lhs, const months& __rhs) noexcept
+{ return __lhs + -__rhs; }
+
+inline constexpr
+months operator-(const month& __lhs, const month& __rhs) noexcept
+{
+    auto const __dm = static_cast<unsigned>(__lhs) - static_cast<unsigned>(__rhs);
+    return months(__dm <= 11 ? __dm : __dm + 12);
+}
+
+inline constexpr month& month::operator+=(const months& __dm) noexcept
+{ *this = *this + __dm; return *this; }
+
+inline constexpr month& month::operator-=(const months& __dm) noexcept
+{ *this = *this - __dm; return *this; }
+
+
+class _LIBCPP_TYPE_VIS year {
+private:
+    short __y;
+public:
+    year() = default;
+    explicit inline constexpr year(int __val) noexcept : __y(static_cast<short>(__val)) {}
+
+    inline constexpr year& operator++()    noexcept { ++__y; return *this; };
+    inline constexpr year  operator++(int) noexcept { year __tmp = *this; ++(*this); return __tmp; };
+    inline constexpr year& operator--()    noexcept { --__y; return *this; };
+    inline constexpr year  operator--(int) noexcept { year __tmp = *this; --(*this); return __tmp; };
+           constexpr year& operator+=(const years& __dy) noexcept;
+           constexpr year& operator-=(const years& __dy) noexcept;
+    inline constexpr year operator+() const noexcept { return *this; }
+    inline constexpr year operator-() const noexcept { return year{-__y}; };
+
+    inline constexpr bool is_leap() const noexcept { return __y % 4 == 0 && (__y % 100 != 0 || __y % 400 == 0); }
+    explicit inline constexpr operator int() const noexcept { return __y; }
+           constexpr bool ok() const noexcept;
+    static inline constexpr year min() noexcept { return year{-32767}; }
+    static inline constexpr year max() noexcept { return year{ 32767}; }
+};
+
+
+inline constexpr
+bool operator==(const year& __lhs, const year& __rhs) noexcept
+{ return static_cast<int>(__lhs) == static_cast<int>(__rhs); }
+
+inline constexpr
+bool operator!=(const year& __lhs, const year& __rhs) noexcept
+{ return !(__lhs == __rhs); }
+
+inline constexpr
+bool operator< (const year& __lhs, const year& __rhs) noexcept
+{ return static_cast<int>(__lhs)  < static_cast<int>(__rhs); }
+
+inline constexpr
+bool operator> (const year& __lhs, const year& __rhs) noexcept
+{ return __rhs < __lhs; }
+
+inline constexpr
+bool operator<=(const year& __lhs, const year& __rhs) noexcept
+{ return !(__rhs < __lhs); }
+
+inline constexpr
+bool operator>=(const year& __lhs, const year& __rhs) noexcept
+{ return !(__lhs < __rhs); }
+
+inline constexpr
+year operator+ (const year& __lhs, const years& __rhs) noexcept
+{ return year(static_cast<int>(__lhs) + __rhs.count()); }
+
+inline constexpr
+year operator+ (const years& __lhs, const year& __rhs) noexcept
+{ return __rhs + __lhs; }
+
+inline constexpr
+year operator- (const year& __lhs, const years& __rhs) noexcept
+{ return __lhs + -__rhs; }
+
+inline constexpr
+years operator-(const year& __lhs, const year& __rhs) noexcept
+{ return years{static_cast<int>(__lhs) - static_cast<int>(__rhs)}; }
+
+
+inline constexpr year& year::operator+=(const years& __dy) noexcept
+{ *this = *this + __dy; return *this; }
+
+inline constexpr year& year::operator-=(const years& __dy) noexcept
+{ *this = *this - __dy; return *this; }
+
+inline constexpr bool year::ok() const noexcept
+{ return static_cast<int>(min()) <= __y && __y <= static_cast<int>(max()); }
+
+class _LIBCPP_TYPE_VIS weekday_indexed;
+class _LIBCPP_TYPE_VIS weekday_last;
+
+class _LIBCPP_TYPE_VIS weekday {
+private:
+    unsigned char __wd;
+public:
+  weekday() = default;
+  inline explicit constexpr weekday(unsigned __val) noexcept : __wd(static_cast<unsigned char>(__val)) {}
+  inline constexpr          weekday(const sys_days& __sysd) noexcept
+          : __wd(__weekday_from_days(__sysd.time_since_epoch().count())) {}
+  inline explicit constexpr weekday(const local_days& __locd) noexcept
+          : __wd(__weekday_from_days(__locd.time_since_epoch().count())) {}
+
+  inline constexpr weekday& operator++()    noexcept { __wd = (__wd == 6 ? 0 : __wd + 1); return *this; }
+  inline constexpr weekday  operator++(int) noexcept { weekday __tmp = *this; ++(*this); return __tmp; }
+  inline constexpr weekday& operator--()    noexcept { __wd = (__wd == 0 ? 6 : __wd - 1); return *this; }
+  inline constexpr weekday  operator--(int) noexcept { weekday __tmp = *this; --(*this); return __tmp; }
+         constexpr weekday& operator+=(const days& __dd) noexcept;
+         constexpr weekday& operator-=(const days& __dd) noexcept;
+  inline explicit constexpr operator unsigned() const noexcept { return __wd; }
+  inline constexpr bool ok() const noexcept { return __wd <= 6; }
+         constexpr weekday_indexed operator[](unsigned __index) const noexcept;
+         constexpr weekday_last    operator[](last_spec) const noexcept;
+
+  static constexpr unsigned char __weekday_from_days(int __days) noexcept;
+};
+
+
+// https://howardhinnant.github.io/date_algorithms.html#weekday_from_days
+inline constexpr
+unsigned char weekday::__weekday_from_days(int __days) noexcept
+{
+    return static_cast<unsigned char>(
+              static_cast<unsigned>(__days >= -4 ? (__days+4) % 7 : (__days+5) % 7 + 6)
+           );
+}
+
+inline constexpr
+bool operator==(const weekday& __lhs, const weekday& __rhs) noexcept
+{ return static_cast<unsigned>(__lhs) == static_cast<unsigned>(__rhs); }
+
+inline constexpr
+bool operator!=(const weekday& __lhs, const weekday& __rhs) noexcept
+{ return !(__lhs == __rhs); }
+
+inline constexpr
+bool operator< (const weekday& __lhs, const weekday& __rhs) noexcept
+{ return static_cast<unsigned>(__lhs) <  static_cast<unsigned>(__rhs); }
+
+inline constexpr
+bool operator> (const weekday& __lhs, const weekday& __rhs) noexcept
+{ return __rhs < __lhs; }
+
+inline constexpr
+bool operator<=(const weekday& __lhs, const weekday& __rhs) noexcept
+{ return !(__rhs < __lhs);}
+
+inline constexpr
+bool operator>=(const weekday& __lhs, const weekday& __rhs) noexcept
+{ return !(__lhs < __rhs); }
+
+constexpr weekday operator+(const weekday& __lhs, const days& __rhs) noexcept
+{
+    auto const __mu = static_cast<long long>(static_cast<unsigned>(__lhs)) + __rhs.count();
+    auto const __yr = (__mu >= 0 ? __mu : __mu - 6) / 7;
+    return weekday{static_cast<unsigned>(__mu - __yr * 7)};
+}
+
+constexpr weekday operator+(const days& __lhs, const weekday& __rhs) noexcept
+{ return __rhs + __lhs; }
+
+constexpr weekday operator-(const weekday& __lhs, const days& __rhs) noexcept
+{ return __lhs + -__rhs; }
+
+constexpr days operator-(const weekday& __lhs, const weekday& __rhs) noexcept
+{
+    const int __wdu = static_cast<unsigned>(__lhs) - static_cast<unsigned>(__rhs);
+    const int __wk = (__wdu >= 0 ? __wdu : __wdu-6) / 7;
+    return days{__wdu - __wk * 7};
+}
+
+inline constexpr weekday& weekday::operator+=(const days& __dd) noexcept
+{ *this = *this + __dd; return *this; }
+
+inline constexpr weekday& weekday::operator-=(const days& __dd) noexcept
+{ *this = *this - __dd; return *this; }
+
+
+class _LIBCPP_TYPE_VIS weekday_indexed {
+private:
+    _VSTD::chrono::weekday __wd;
+    unsigned char          __idx;
+public:
+    weekday_indexed() = default;
+    inline constexpr weekday_indexed(const _VSTD::chrono::weekday& __wdval, unsigned __idxval) noexcept
+        : __wd{__wdval}, __idx(__idxval) {}
+    inline constexpr _VSTD::chrono::weekday weekday() const noexcept { return __wd; }
+    inline constexpr unsigned                 index() const noexcept { return __idx; }
+    inline constexpr bool ok() const noexcept { return __wd.ok() && __idx >= 1 && __idx <= 5; }
+};
+
+inline constexpr
+bool operator==(const weekday_indexed& __lhs, const weekday_indexed& __rhs) noexcept
+{ return __lhs.weekday() == __rhs.weekday() && __lhs.index() == __rhs.index(); }
+
+inline constexpr
+bool operator!=(const weekday_indexed& __lhs, const weekday_indexed& __rhs) noexcept
+{ return !(__lhs == __rhs); }
+
+
+class _LIBCPP_TYPE_VIS weekday_last {
+private:
+    _VSTD::chrono::weekday __wd;
+public:
+    explicit constexpr weekday_last(const _VSTD::chrono::weekday& __val) noexcept
+        : __wd{__val} {}
+    constexpr _VSTD::chrono::weekday weekday() const noexcept { return __wd; }
+    constexpr bool ok() const noexcept { return __wd.ok(); }
+};
+
+inline constexpr
+bool operator==(const weekday_last& __lhs, const weekday_last& __rhs) noexcept
+{ return __lhs.weekday() == __rhs.weekday(); }
+
+inline constexpr
+bool operator!=(const weekday_last& __lhs, const weekday_last& __rhs) noexcept
+{ return !(__lhs == __rhs); }
+
+inline constexpr
+weekday_indexed weekday::operator[](unsigned __index) const noexcept { return weekday_indexed{*this, __index}; }
+
+inline constexpr 
+weekday_last    weekday::operator[](last_spec) const noexcept { return weekday_last{*this}; }
+
+
+inline constexpr last_spec last{};
+inline constexpr weekday   Sunday{0};
+inline constexpr weekday   Monday{1};
+inline constexpr weekday   Tuesday{2};
+inline constexpr weekday   Wednesday{3};
+inline constexpr weekday   Thursday{4};
+inline constexpr weekday   Friday{5};
+inline constexpr weekday   Saturday{6};
+
+inline constexpr month January{1};
+inline constexpr month February{2};
+inline constexpr month March{3};
+inline constexpr month April{4};
+inline constexpr month May{5};
+inline constexpr month June{6};
+inline constexpr month July{7};
+inline constexpr month August{8};
+inline constexpr month September{9};
+inline constexpr month October{10};
+inline constexpr month November{11};
+inline constexpr month December{12};
+
+
+class _LIBCPP_TYPE_VIS month_day {
+private:
+   chrono::month __m;
+   chrono::day   __d;
+public:
+    month_day() = default;
+    constexpr month_day(const chrono::month& __mval, const chrono::day& __dval) noexcept
+        : __m{__mval}, __d{__dval} {}
+    inline constexpr chrono::month month() const noexcept { return __m; }
+    inline constexpr chrono::day   day()   const noexcept { return __d; }
+    constexpr bool ok() const noexcept;
+};
+
+inline constexpr
+bool month_day::ok() const noexcept
+{
+    if (!__m.ok()) return false;
+    const unsigned __dval = static_cast<unsigned>(__d);
+    if (__dval < 1 || __dval > 31) return false;
+    if (__dval <= 29) return true;
+//  Now we've got either 30 or 31
+    const unsigned __mval = static_cast<unsigned>(__m);
+    if (__mval == 2) return false;
+    if (__mval == 4 || __mval == 6 || __mval == 9 || __mval == 11)
+        return __dval == 30;
+    return true;
+}
+
+inline constexpr
+bool operator==(const month_day& __lhs, const month_day& __rhs) noexcept
+{ return __lhs.month() == __rhs.month() && __lhs.day() == __rhs.day(); }
+
+inline constexpr
+bool operator!=(const month_day& __lhs, const month_day& __rhs) noexcept
+{ return !(__lhs == __rhs); }
+
+inline constexpr
+month_day operator/(const month& __lhs, const day& __rhs) noexcept
+{ return month_day{__lhs, __rhs}; }
+
+constexpr
+month_day operator/(const day& __lhs, const month& __rhs) noexcept
+{ return __rhs / __lhs; }
+
+inline constexpr
+month_day operator/(const month& __lhs, int __rhs) noexcept
+{ return __lhs / day(__rhs); }
+
+constexpr
+month_day operator/(int __lhs, const day& __rhs) noexcept
+{ return month(__lhs) / __rhs; }
+
+constexpr
+month_day operator/(const day& __lhs, int __rhs) noexcept
+{ return month(__rhs) / __lhs; }
+
+
+inline constexpr
+bool operator< (const month_day& __lhs, const month_day& __rhs) noexcept
+{ return __lhs.month() != __rhs.month() ? __lhs.month() < __rhs.month() : __lhs.day() < __rhs.day(); }
+
+inline constexpr
+bool operator> (const month_day& __lhs, const month_day& __rhs) noexcept
+{ return __rhs < __lhs; }
+
+inline constexpr
+bool operator<=(const month_day& __lhs, const month_day& __rhs) noexcept
+{ return !(__rhs < __lhs);}
+
+inline constexpr
+bool operator>=(const month_day& __lhs, const month_day& __rhs) noexcept
+{ return !(__lhs < __rhs); }
+
+
+
+class _LIBCPP_TYPE_VIS month_day_last {
+private:
+    chrono::month __m;
+public:
+    explicit constexpr month_day_last(const chrono::month& __val) noexcept
+        : __m{__val} {}
+    inline constexpr chrono::month month() const noexcept { return __m; }
+    inline constexpr bool ok() const noexcept { return __m.ok(); }
+};
+
+inline constexpr
+bool operator==(const month_day_last& __lhs, const month_day_last& __rhs) noexcept
+{ return __lhs.month() == __rhs.month(); }
+
+inline constexpr
+bool operator!=(const month_day_last& __lhs, const month_day_last& __rhs) noexcept
+{ return !(__lhs == __rhs); }
+
+inline constexpr
+bool operator< (const month_day_last& __lhs, const month_day_last& __rhs) noexcept
+{ return __lhs.month() < __rhs.month(); }
+
+inline constexpr
+bool operator> (const month_day_last& __lhs, const month_day_last& __rhs) noexcept
+{ return __rhs < __lhs; }
+
+inline constexpr
+bool operator<=(const month_day_last& __lhs, const month_day_last& __rhs) noexcept
+{ return !(__rhs < __lhs);}
+
+inline constexpr
+bool operator>=(const month_day_last& __lhs, const month_day_last& __rhs) noexcept
+{ return !(__lhs < __rhs); }
+
+inline constexpr
+month_day_last operator/(const month& __lhs, last_spec) noexcept
+{ return month_day_last{__lhs}; }
+
+inline constexpr
+month_day_last operator/(last_spec, const month& __rhs) noexcept
+{ return month_day_last{__rhs}; }
+
+inline constexpr
+month_day_last operator/(int __lhs, last_spec) noexcept
+{ return month_day_last{month(__lhs)}; }
+
+inline constexpr
+month_day_last operator/(last_spec, int __rhs) noexcept
+{ return month_day_last{month(__rhs)}; }
+
+
+class _LIBCPP_TYPE_VIS month_weekday {
+private:
+    chrono::month __m;
+    chrono::weekday_indexed __wdi;
+public:
+    month_weekday() = default;
+    constexpr month_weekday(const chrono::month& __mval, const chrono::weekday_indexed& __wdival) noexcept
+        : __m{__mval}, __wdi{__wdival} {}
+    inline constexpr chrono::month                     month() const noexcept { return __m; }
+    inline constexpr chrono::weekday_indexed weekday_indexed() const noexcept { return __wdi; }
+    inline constexpr bool                                 ok() const noexcept { return __m.ok() && __wdi.ok(); }
+};
+
+inline constexpr
+bool operator==(const month_weekday& __lhs, const month_weekday& __rhs) noexcept
+{ return __lhs.month() == __rhs.month() && __lhs.weekday_indexed() == __rhs.weekday_indexed(); }
+
+inline constexpr
+bool operator!=(const month_weekday& __lhs, const month_weekday& __rhs) noexcept
+{ return !(__lhs == __rhs); }
+
+inline constexpr
+month_weekday operator/(const month& __lhs, const weekday_indexed& __rhs) noexcept
+{ return month_weekday{__lhs, __rhs}; }
+
+inline constexpr
+month_weekday operator/(int __lhs, const weekday_indexed& __rhs) noexcept
+{ return month_weekday{month(__lhs), __rhs}; }
+
+inline constexpr
+month_weekday operator/(const weekday_indexed& __lhs, const month& __rhs) noexcept
+{ return month_weekday{__rhs, __lhs}; }
+
+inline constexpr
+month_weekday operator/(const weekday_indexed& __lhs, int __rhs) noexcept
+{ return month_weekday{month(__rhs), __lhs}; }
+
+
+class _LIBCPP_TYPE_VIS month_weekday_last {
+    chrono::month        __m;
+    chrono::weekday_last __wdl;
+  public:
+    constexpr month_weekday_last(const chrono::month& __mval, const chrono::weekday_last& __wdlval) noexcept
+        : __m{__mval}, __wdl{__wdlval} {}
+    inline constexpr chrono::month               month() const noexcept { return __m; }
+    inline constexpr chrono::weekday_last weekday_last() const noexcept { return __wdl; }
+    inline constexpr bool                           ok() const noexcept { return __m.ok() && __wdl.ok(); }
+};
+
+inline constexpr
+bool operator==(const month_weekday_last& __lhs, const month_weekday_last& __rhs) noexcept
+{ return __lhs.month() == __rhs.month() && __lhs.weekday_last() == __rhs.weekday_last(); }
+
+inline constexpr
+bool operator!=(const month_weekday_last& __lhs, const month_weekday_last& __rhs) noexcept
+{ return !(__lhs == __rhs); }
+
+
+inline constexpr
+month_weekday_last operator/(const month& __lhs, const weekday_last& __rhs) noexcept
+{ return month_weekday_last{__lhs, __rhs}; }
+
+inline constexpr
+month_weekday_last operator/(int __lhs, const weekday_last& __rhs) noexcept
+{ return month_weekday_last{month(__lhs), __rhs}; }
+
+inline constexpr
+month_weekday_last operator/(const weekday_last& __lhs, const month& __rhs) noexcept
+{ return month_weekday_last{__rhs, __lhs}; }
+
+inline constexpr
+month_weekday_last operator/(const weekday_last& __lhs, int __rhs) noexcept
+{ return month_weekday_last{month(__rhs), __lhs}; }
+
+
+class _LIBCPP_TYPE_VIS year_month {
+    chrono::year  __y;
+    chrono::month __m;
+public:
+    year_month() = default;
+    constexpr year_month(const chrono::year& __yval, const chrono::month& __mval) noexcept
+        : __y{__yval}, __m{__mval} {}
+    inline constexpr chrono::year  year()  const noexcept { return __y; }
+    inline constexpr chrono::month month() const noexcept { return __m; }
+    inline constexpr year_month& operator+=(const months& __dm) noexcept { this->__m += __dm; return *this; }
+    inline constexpr year_month& operator-=(const months& __dm) noexcept { this->__m -= __dm; return *this; }
+    inline constexpr year_month& operator+=(const years& __dy)  noexcept { this->__y += __dy; return *this; }
+    inline constexpr year_month& operator-=(const years& __dy)  noexcept { this->__y -= __dy; return *this; }
+    inline constexpr bool ok() const noexcept { return __y.ok() && __m.ok(); }
+};
+
+inline constexpr
+year_month operator/(const year& __y, const month& __m) noexcept { return year_month{__y, __m}; }
+
+inline constexpr
+year_month operator/(const year& __y, int __m) noexcept { return year_month{__y, month(__m)}; }
+
+inline constexpr
+bool operator==(const year_month& __lhs, const year_month& __rhs) noexcept
+{ return __lhs.year() == __rhs.year() && __lhs.month() == __rhs.month(); }
+
+inline constexpr
+bool operator!=(const year_month& __lhs, const year_month& __rhs) noexcept
+{ return !(__lhs == __rhs); }
+
+inline constexpr
+bool operator< (const year_month& __lhs, const year_month& __rhs) noexcept
+{ return __lhs.year() != __rhs.year() ? __lhs.year() < __rhs.year() : __lhs.month() < __rhs.month(); }
+
+inline constexpr
+bool operator> (const year_month& __lhs, const year_month& __rhs) noexcept
+{ return __rhs < __lhs; }
+
+inline constexpr
+bool operator<=(const year_month& __lhs, const year_month& __rhs) noexcept
+{ return !(__rhs < __lhs);}
+
+inline constexpr
+bool operator>=(const year_month& __lhs, const year_month& __rhs) noexcept
+{ return !(__lhs < __rhs); }
+
+constexpr year_month operator+(const year_month& __lhs, const months& __rhs) noexcept
+{
+    int __dmi = static_cast<int>(static_cast<unsigned>(__lhs.month())) - 1 + __rhs.count();
+    const int __dy = (__dmi >= 0 ? __dmi : __dmi-11) / 12;
+    __dmi = __dmi - __dy * 12 + 1;
+    return (__lhs.year() + years(__dy)) / month(static_cast<unsigned>(__dmi));
+}
+
+constexpr year_month operator+(const months& __lhs, const year_month& __rhs) noexcept
+{ return __rhs + __lhs; }
+
+constexpr year_month operator+(const year_month& __lhs, const years& __rhs) noexcept
+{ return (__lhs.year() + __rhs) / __lhs.month(); }
+
+constexpr year_month operator+(const years& __lhs, const year_month& __rhs) noexcept
+{ return __rhs + __lhs; }
+
+constexpr months     operator-(const year_month& __lhs, const year_month& __rhs) noexcept
+{ return (__lhs.year() - __rhs.year()) + months(static_cast<unsigned>(__lhs.month()) - static_cast<unsigned>(__rhs.month())); }
+
+constexpr year_month operator-(const year_month& __lhs, const months& __rhs) noexcept
+{ return __lhs + -__rhs; }
+
+constexpr year_month operator-(const year_month& __lhs, const years& __rhs) noexcept
+{ return __lhs + -__rhs; }
+
+class year_month_day_last;
+
+class _LIBCPP_TYPE_VIS year_month_day {
+private:
+    chrono::year  __y;
+    chrono::month __m;
+    chrono::day   __d;
+public:
+     year_month_day() = default;
+     inline constexpr year_month_day(
+            const chrono::year& __yval, const chrono::month& __mval, const chrono::day& __dval) noexcept
+            : __y{__yval}, __m{__mval}, __d{__dval} {}  
+            constexpr year_month_day(const year_month_day_last& __ymdl) noexcept;
+     inline constexpr year_month_day(const sys_days& __sysd) noexcept
+            : year_month_day(__from_days(__sysd.time_since_epoch())) {}
+     inline explicit constexpr year_month_day(const local_days& __locd) noexcept
+            : year_month_day(__from_days(__locd.time_since_epoch())) {}
+
+            constexpr year_month_day& operator+=(const months& __dm) noexcept;
+            constexpr year_month_day& operator-=(const months& __dm) noexcept;
+            constexpr year_month_day& operator+=(const years& __dy)  noexcept;
+            constexpr year_month_day& operator-=(const years& __dy)  noexcept;
+
+     inline constexpr chrono::year   year() const noexcept { return __y; }
+     inline constexpr chrono::month month() const noexcept { return __m; }
+     inline constexpr chrono::day     day() const noexcept { return __d; }
+     inline constexpr operator   sys_days() const noexcept          { return   sys_days{__to_days()}; }
+     inline explicit constexpr operator local_days() const noexcept { return local_days{__to_days()}; }
+
+            constexpr bool             ok() const noexcept;
+
+     static constexpr year_month_day __from_days(days __d) noexcept;
+     constexpr days __to_days() const noexcept;
+};
+
+
+// https://howardhinnant.github.io/date_algorithms.html#civil_from_days
+inline constexpr
+year_month_day
+year_month_day::__from_days(days __d) noexcept
+{
+    static_assert(std::numeric_limits<unsigned>::digits >= 18, "");
+    static_assert(std::numeric_limits<int>::digits >= 20     , "");
+    const int      __z = __d.count() + 719468;
+    const int      __era = (__z >= 0 ? __z : __z - 146096) / 146097;
+    const unsigned __doe = static_cast<unsigned>(__z - __era * 146097);              // [0, 146096]
+    const unsigned __yoe = (__doe - __doe/1460 + __doe/36524 - __doe/146096) / 365;  // [0, 399]
+    const int      __yr = static_cast<int>(__yoe) + __era * 400;
+    const unsigned __doy = __doe - (365 * __yoe + __yoe/4 - __yoe/100);              // [0, 365]
+    const unsigned __mp = (5 * __doy + 2)/153;                                       // [0, 11]
+    const unsigned __dy = __doy - (153 * __mp + 2)/5 + 1;                            // [1, 31]
+    const unsigned __mth = __mp + (__mp < 10 ? 3 : -9);                              // [1, 12]
+    return year_month_day{chrono::year{__yr + (__mth <= 2)}, chrono::month{__mth}, chrono::day{__dy}};
+}
+
+// https://howardhinnant.github.io/date_algorithms.html#days_from_civil
+inline constexpr days year_month_day::__to_days() const noexcept
+{
+    static_assert(std::numeric_limits<unsigned>::digits >= 18, "");
+    static_assert(std::numeric_limits<int>::digits >= 20     , "");
+
+    const int      __yr  = static_cast<int>(__y) - (__m <= February);
+    const unsigned __mth = static_cast<unsigned>(__m);
+    const unsigned __dy  = static_cast<unsigned>(__d);
+
+    const int      __era = (__yr >= 0 ? __yr : __yr - 399) / 400;
+    const unsigned __yoe = static_cast<unsigned>(__yr - __era * 400);                // [0, 399]
+    const unsigned __doy = (153 * (__mth + (__mth > 2 ? -3 : 9)) + 2) / 5 + __dy-1;  // [0, 365]
+    const unsigned __doe = __yoe * 365 + __yoe/4 - __yoe/100 + __doy;                // [0, 146096]
+    return days{__era * 146097 + static_cast<int>(__doe) - 719468};
+}
+
+inline constexpr
+bool operator==(const year_month_day& __lhs, const year_month_day& __rhs) noexcept
+{ return __lhs.year() == __rhs.year() && __lhs.month() == __rhs.month() && __lhs.day() == __rhs.day(); }
+
+inline constexpr
+bool operator!=(const year_month_day& __lhs, const year_month_day& __rhs) noexcept
+{ return !(__lhs == __rhs); }
+
+inline constexpr
+bool operator< (const year_month_day& __lhs, const year_month_day& __rhs) noexcept
+{
+    if (__lhs.year() < __rhs.year()) return true;
+    if (__lhs.year() > __rhs.year()) return false;
+    if (__lhs.month() < __rhs.month()) return true;
+    if (__lhs.month() > __rhs.month()) return false;
+    return __lhs.day() < __rhs.day();
+}
+
+inline constexpr
+bool operator> (const year_month_day& __lhs, const year_month_day& __rhs) noexcept
+{ return __rhs < __lhs; }
+
+inline constexpr
+bool operator<=(const year_month_day& __lhs, const year_month_day& __rhs) noexcept
+{ return !(__rhs < __lhs);}
+
+inline constexpr
+bool operator>=(const year_month_day& __lhs, const year_month_day& __rhs) noexcept
+{ return !(__lhs < __rhs); }
+
+inline constexpr
+year_month_day operator/(const year_month& __lhs, const day& __rhs) noexcept
+{ return year_month_day{__lhs.year(), __lhs.month(), __rhs}; }
+
+inline constexpr
+year_month_day operator/(const year_month& __lhs, int __rhs) noexcept
+{ return __lhs / day(__rhs); }
+
+inline constexpr
+year_month_day operator/(const year& __lhs, const month_day& __rhs) noexcept
+{ return __lhs / __rhs.month() / __rhs.day(); }
+
+inline constexpr
+year_month_day operator/(int __lhs, const month_day& __rhs) noexcept
+{ return year(__lhs) / __rhs; }
+
+inline constexpr
+year_month_day operator/(const month_day& __lhs, const year& __rhs) noexcept
+{ return __rhs / __lhs; }
+
+inline constexpr
+year_month_day operator/(const month_day& __lhs, int __rhs) noexcept
+{ return year(__rhs) / __lhs; }
+
+
+inline constexpr
+year_month_day operator+(const year_month_day& __lhs, const months& __rhs) noexcept
+{ return (__lhs.year()/__lhs.month() + __rhs)/__lhs.day(); }
+
+inline constexpr
+year_month_day operator+(const months& __lhs, const year_month_day& __rhs) noexcept
+{ return __rhs + __lhs; }
+
+inline constexpr
+year_month_day operator-(const year_month_day& __lhs, const months& __rhs) noexcept
+{ return __lhs + -__rhs; }
+
+inline constexpr
+year_month_day operator+(const year_month_day& __lhs, const years& __rhs) noexcept
+{ return (__lhs.year() + __rhs) / __lhs.month() / __lhs.day(); }
+
+inline constexpr
+year_month_day operator+(const years& __lhs, const year_month_day& __rhs) noexcept
+{ return __rhs + __lhs; }
+
+inline constexpr
+year_month_day operator-(const year_month_day& __lhs, const years& __rhs) noexcept
+{ return __lhs + -__rhs; }
+
+inline constexpr year_month_day& year_month_day::operator+=(const months& __dm) noexcept { *this = *this + __dm; return *this; }
+inline constexpr year_month_day& year_month_day::operator-=(const months& __dm) noexcept { *this = *this - __dm; return *this; }
+inline constexpr year_month_day& year_month_day::operator+=(const years& __dy)  noexcept { *this = *this + __dy; return *this; }
+inline constexpr year_month_day& year_month_day::operator-=(const years& __dy)  noexcept { *this = *this - __dy; return *this; }
+
+class _LIBCPP_TYPE_VIS year_month_day_last {
+private:
+    chrono::year           __y;
+    chrono::month_day_last __mdl;
+public:
+     constexpr year_month_day_last(const year& __yval, const month_day_last& __mdlval) noexcept
+        : __y{__yval}, __mdl{__mdlval} {}
+
+     constexpr year_month_day_last& operator+=(const months& __m) noexcept;
+     constexpr year_month_day_last& operator-=(const months& __m) noexcept;
+     constexpr year_month_day_last& operator+=(const years& __y)  noexcept;
+     constexpr year_month_day_last& operator-=(const years& __y)  noexcept;
+
+     inline constexpr chrono::year                     year() const noexcept { return __y; }
+     inline constexpr chrono::month                   month() const noexcept { return __mdl.month(); }
+     inline constexpr chrono::month_day_last month_day_last() const noexcept { return __mdl; }
+            constexpr chrono::day                       day() const noexcept;
+     inline constexpr operator                     sys_days() const noexcept { return   sys_days{year()/month()/day()}; }
+     inline explicit constexpr operator          local_days() const noexcept { return local_days{year()/month()/day()}; }
+     inline constexpr bool                               ok() const noexcept { return __y.ok() && __mdl.ok(); }
+};
+
+inline constexpr
+chrono::day year_month_day_last::day() const noexcept
+{
+    constexpr chrono::day __d[] =
+    {
+        chrono::day(31), chrono::day(28), chrono::day(31),
+        chrono::day(30), chrono::day(31), chrono::day(30),
+        chrono::day(31), chrono::day(31), chrono::day(30),
+        chrono::day(31), chrono::day(30), chrono::day(31)
+    };
+    return month() != February || !__y.is_leap() ?
+        __d[static_cast<unsigned>(month()) - 1] : chrono::day{29};
+}
+
+inline constexpr
+bool operator==(const year_month_day_last& __lhs, const year_month_day_last& __rhs) noexcept
+{ return __lhs.year() == __rhs.year() && __lhs.month_day_last() == __rhs.month_day_last(); }
+
+inline constexpr
+bool operator!=(const year_month_day_last& __lhs, const year_month_day_last& __rhs) noexcept
+{ return !(__lhs == __rhs); }
+
+inline constexpr
+bool operator< (const year_month_day_last& __lhs, const year_month_day_last& __rhs) noexcept
+{
+    if (__lhs.year() < __rhs.year()) return true;
+    if (__lhs.year() > __rhs.year()) return false;
+    return __lhs.month_day_last() < __rhs.month_day_last();
+}
+
+inline constexpr
+bool operator> (const year_month_day_last& __lhs, const year_month_day_last& __rhs) noexcept
+{ return __rhs < __lhs; }
+
+inline constexpr
+bool operator<=(const year_month_day_last& __lhs, const year_month_day_last& __rhs) noexcept
+{ return !(__rhs < __lhs);}
+
+inline constexpr
+bool operator>=(const year_month_day_last& __lhs, const year_month_day_last& __rhs) noexcept
+{ return !(__lhs < __rhs); }
+
+inline constexpr year_month_day_last operator/(const year_month& __lhs, last_spec) noexcept
+{ return year_month_day_last{__lhs.year(), month_day_last{__lhs.month()}}; }
+
+inline constexpr year_month_day_last operator/(const year& __lhs, const month_day_last& __rhs) noexcept
+{ return year_month_day_last{__lhs, __rhs}; }
+
+inline constexpr year_month_day_last operator/(int __lhs, const month_day_last& __rhs) noexcept
+{ return year_month_day_last{year{__lhs}, __rhs}; }
+
+inline constexpr year_month_day_last operator/(const month_day_last& __lhs, const year& __rhs) noexcept
+{ return __rhs / __lhs; }
+
+inline constexpr year_month_day_last operator/(const month_day_last& __lhs, int __rhs) noexcept
+{ return year{__rhs} / __lhs; }
+
+
+inline constexpr
+year_month_day_last operator+(const year_month_day_last& __lhs, const months& __rhs) noexcept
+{ return (__lhs.year() / __lhs.month() + __rhs) / last; }
+
+inline constexpr
+year_month_day_last operator+(const months& __lhs, const year_month_day_last& __rhs) noexcept
+{ return __rhs + __lhs; }
+
+inline constexpr
+year_month_day_last operator-(const year_month_day_last& __lhs, const months& __rhs) noexcept
+{ return __lhs + (-__rhs); }
+
+inline constexpr
+year_month_day_last operator+(const year_month_day_last& __lhs, const years& __rhs) noexcept
+{ return year_month_day_last{__lhs.year() + __rhs, __lhs.month_day_last()}; }
+
+inline constexpr
+year_month_day_last operator+(const years& __lhs, const year_month_day_last& __rhs) noexcept
+{ return __rhs + __lhs; }
+
+inline constexpr
+year_month_day_last operator-(const year_month_day_last& __lhs, const years& __rhs) noexcept
+{ return __lhs + (-__rhs); }
+
+inline constexpr year_month_day_last& year_month_day_last::operator+=(const months& __dm) noexcept { *this = *this + __dm; return *this; }
+inline constexpr year_month_day_last& year_month_day_last::operator-=(const months& __dm) noexcept { *this = *this - __dm; return *this; }
+inline constexpr year_month_day_last& year_month_day_last::operator+=(const years& __dy)  noexcept { *this = *this + __dy; return *this; }
+inline constexpr year_month_day_last& year_month_day_last::operator-=(const years& __dy)  noexcept { *this = *this - __dy; return *this; }
+
+inline constexpr year_month_day::year_month_day(const year_month_day_last& __ymdl) noexcept
+    : __y{__ymdl.year()}, __m{__ymdl.month()}, __d{__ymdl.day()} {}  
+
+inline constexpr bool year_month_day::ok() const noexcept
+{
+    if (!__y.ok() || !__m.ok()) return false;
+    return chrono::day{1} <= __d && __d <= (__y / __m / last).day();
+}
+
+class _LIBCPP_TYPE_VIS year_month_weekday {
+    chrono::year            __y;
+    chrono::month           __m;
+    chrono::weekday_indexed __wdi;
+public:
+    year_month_weekday() = default;
+    constexpr year_month_weekday(const chrono::year& __yval, const chrono::month& __mval,
+                               const chrono::weekday_indexed& __wdival) noexcept
+        : __y{__yval}, __m{__mval}, __wdi{__wdival} {}
+    constexpr year_month_weekday(const sys_days& __sysd) noexcept
+            : year_month_weekday(__from_days(__sysd.time_since_epoch())) {}
+    inline explicit constexpr year_month_weekday(const local_days& __locd) noexcept
+            : year_month_weekday(__from_days(__locd.time_since_epoch())) {}
+    constexpr year_month_weekday& operator+=(const months& m) noexcept;
+    constexpr year_month_weekday& operator-=(const months& m) noexcept;
+    constexpr year_month_weekday& operator+=(const years& y)  noexcept;
+    constexpr year_month_weekday& operator-=(const years& y)  noexcept;
+
+    inline constexpr chrono::year                       year() const noexcept { return __y; }
+    inline constexpr chrono::month                     month() const noexcept { return __m; }
+    inline constexpr chrono::weekday                 weekday() const noexcept { return __wdi.weekday(); }
+    inline constexpr unsigned                          index() const noexcept { return __wdi.index(); }
+    inline constexpr chrono::weekday_indexed weekday_indexed() const noexcept { return __wdi; }
+
+    inline constexpr                       operator sys_days() const noexcept { return   sys_days{__to_days()}; }
+    inline explicit constexpr operator            local_days() const noexcept { return local_days{__to_days()}; }
+    inline constexpr bool ok() const noexcept
+    {
+        if (!__y.ok() || !__m.ok() || !__wdi.ok()) return false;
+    //  TODO: make sure it's a valid date
+        return true;
+    }
+
+    static constexpr year_month_weekday __from_days(days __d) noexcept;
+    constexpr days __to_days() const noexcept;
+};
+
+inline constexpr
+year_month_weekday year_month_weekday::__from_days(days __d) noexcept
+{
+    const sys_days      __sysd{__d};
+    const chrono::weekday __wd = chrono::weekday(__sysd);
+    const year_month_day __ymd = year_month_day(__sysd);
+    return year_month_weekday{__ymd.year(), __ymd.month(), 
+                              __wd[(static_cast<unsigned>(__ymd.day())-1)/7+1]};
+}
+
+inline constexpr
+days year_month_weekday::__to_days() const noexcept
+{
+    const sys_days __sysd = sys_days(__y/__m/1);
+    return (__sysd + (__wdi.weekday() - chrono::weekday(__sysd) + days{(__wdi.index()-1)*7}))
+                .time_since_epoch();
+}
+
+inline constexpr
+bool operator==(const year_month_weekday& __lhs, const year_month_weekday& __rhs) noexcept
+{ return __lhs.year() == __rhs.year() && __lhs.month() == __rhs.month() && __lhs.weekday_indexed() == __rhs.weekday_indexed(); }
+
+inline constexpr
+bool operator!=(const year_month_weekday& __lhs, const year_month_weekday& __rhs) noexcept
+{ return !(__lhs == __rhs); }
+
+inline constexpr
+year_month_weekday operator/(const year_month& __lhs, const weekday_indexed& __rhs) noexcept
+{ return year_month_weekday{__lhs.year(), __lhs.month(), __rhs}; }
+
+inline constexpr
+year_month_weekday operator/(const year& __lhs, const month_weekday& __rhs) noexcept
+{ return year_month_weekday{__lhs, __rhs.month(), __rhs.weekday_indexed()}; }
+
+inline constexpr
+year_month_weekday operator/(int __lhs, const month_weekday& __rhs) noexcept
+{ return year(__lhs) / __rhs; }
+
+inline constexpr
+year_month_weekday operator/(const month_weekday& __lhs, const year& __rhs) noexcept
+{ return __rhs / __lhs; }
+
+inline constexpr
+year_month_weekday operator/(const month_weekday& __lhs, int __rhs) noexcept
+{ return year(__rhs) / __lhs; }
+
+
+inline constexpr
+year_month_weekday operator+(const year_month_weekday& __lhs, const months& __rhs) noexcept
+{ return (__lhs.year() / __lhs.month() + __rhs) / __lhs.weekday_indexed(); }
+
+inline constexpr
+year_month_weekday operator+(const months& __lhs, const year_month_weekday& __rhs) noexcept
+{ return __rhs + __lhs; }
+
+inline constexpr
+year_month_weekday operator-(const year_month_weekday& __lhs, const months& __rhs) noexcept
+{ return __lhs + (-__rhs); }
+
+inline constexpr
+year_month_weekday operator+(const year_month_weekday& __lhs, const years& __rhs) noexcept
+{ return year_month_weekday{__lhs.year() + __rhs, __lhs.month(), __lhs.weekday_indexed()}; }
+
+inline constexpr
+year_month_weekday operator+(const years& __lhs, const year_month_weekday& __rhs) noexcept
+{ return __rhs + __lhs; }
+
+inline constexpr
+year_month_weekday operator-(const year_month_weekday& __lhs, const years& __rhs) noexcept
+{ return __lhs + (-__rhs); }
+
+
+inline constexpr year_month_weekday& year_month_weekday::operator+=(const months& __dm) noexcept { *this = *this + __dm; return *this; }
+inline constexpr year_month_weekday& year_month_weekday::operator-=(const months& __dm) noexcept { *this = *this - __dm; return *this; }
+inline constexpr year_month_weekday& year_month_weekday::operator+=(const years& __dy)  noexcept { *this = *this + __dy; return *this; }
+inline constexpr year_month_weekday& year_month_weekday::operator-=(const years& __dy)  noexcept { *this = *this - __dy; return *this; }
+
+class _LIBCPP_TYPE_VIS year_month_weekday_last {
+private:
+    chrono::year         __y;
+    chrono::month        __m;
+    chrono::weekday_last __wdl;
+public:
+    constexpr year_month_weekday_last(const chrono::year& __yval, const chrono::month& __mval,
+                                      const chrono::weekday_last& __wdlval) noexcept
+                : __y{__yval}, __m{__mval}, __wdl{__wdlval} {}
+    constexpr year_month_weekday_last& operator+=(const months& __dm) noexcept;
+    constexpr year_month_weekday_last& operator-=(const months& __dm) noexcept;
+    constexpr year_month_weekday_last& operator+=(const years& __dy)  noexcept;
+    constexpr year_month_weekday_last& operator-=(const years& __dy)  noexcept;
+
+    inline constexpr chrono::year                 year() const noexcept { return __y; }
+    inline constexpr chrono::month               month() const noexcept { return __m; }
+    inline constexpr chrono::weekday           weekday() const noexcept { return __wdl.weekday(); }
+    inline constexpr chrono::weekday_last weekday_last() const noexcept { return __wdl; }
+    inline constexpr operator                 sys_days() const noexcept { return   sys_days{__to_days()}; }
+    inline explicit constexpr operator      local_days() const noexcept { return local_days{__to_days()}; }
+    inline constexpr bool ok() const noexcept { return __y.ok() && __m.ok() && __wdl.ok(); }
+    
+    constexpr days __to_days() const noexcept;
+    
+};
+
+inline constexpr
+days year_month_weekday_last::__to_days() const noexcept
+{
+    const sys_days __last = sys_days{__y/__m/last};
+    return (__last - (chrono::weekday{__last} - __wdl.weekday())).time_since_epoch();
+
+}
+
+inline constexpr
+bool operator==(const year_month_weekday_last& __lhs, const year_month_weekday_last& __rhs) noexcept
+{ return __lhs.year() == __rhs.year() && __lhs.month() == __rhs.month() && __lhs.weekday_last() == __rhs.weekday_last(); }
+
+inline constexpr
+bool operator!=(const year_month_weekday_last& __lhs, const year_month_weekday_last& __rhs) noexcept
+{ return !(__lhs == __rhs); }
+
+
+inline constexpr
+year_month_weekday_last operator/(const year_month& __lhs, const weekday_last& __rhs) noexcept
+{ return year_month_weekday_last{__lhs.year(), __lhs.month(), __rhs}; }
+
+inline constexpr
+year_month_weekday_last operator/(const year& __lhs, const month_weekday_last& __rhs) noexcept
+{ return year_month_weekday_last{__lhs, __rhs.month(), __rhs.weekday_last()}; }
+
+inline constexpr
+year_month_weekday_last operator/(int __lhs, const month_weekday_last& __rhs) noexcept
+{ return year(__lhs) / __rhs; }
+
+inline constexpr
+year_month_weekday_last operator/(const month_weekday_last& __lhs, const year& __rhs) noexcept
+{ return __rhs / __lhs; }
+
+inline constexpr
+year_month_weekday_last operator/(const month_weekday_last& __lhs, int __rhs) noexcept
+{ return year(__rhs) / __lhs; }  
+
+
+inline constexpr
+year_month_weekday_last operator+(const year_month_weekday_last& __lhs, const months& __rhs) noexcept
+{ return (__lhs.year() / __lhs.month() + __rhs) / __lhs.weekday_last(); }
+
+inline constexpr
+year_month_weekday_last operator+(const months& __lhs, const year_month_weekday_last& __rhs) noexcept
+{ return __rhs + __lhs; }
+
+inline constexpr
+year_month_weekday_last operator-(const year_month_weekday_last& __lhs, const months& __rhs) noexcept
+{ return __lhs + (-__rhs); }
+
+inline constexpr
+year_month_weekday_last operator+(const year_month_weekday_last& __lhs, const years& __rhs) noexcept
+{ return year_month_weekday_last{__lhs.year() + __rhs, __lhs.month(), __lhs.weekday_last()}; }
+
+inline constexpr
+year_month_weekday_last operator+(const years& __lhs, const year_month_weekday_last& __rhs) noexcept
+{ return __rhs + __lhs; }
+
+inline constexpr
+year_month_weekday_last operator-(const year_month_weekday_last& __lhs, const years& __rhs) noexcept
+{ return __lhs + (-__rhs); }
+
+inline constexpr year_month_weekday_last& year_month_weekday_last::operator+=(const months& __dm) noexcept { *this = *this + __dm; return *this; }
+inline constexpr year_month_weekday_last& year_month_weekday_last::operator-=(const months& __dm) noexcept { *this = *this - __dm; return *this; }
+inline constexpr year_month_weekday_last& year_month_weekday_last::operator+=(const years& __dy)  noexcept { *this = *this + __dy; return *this; }
+inline constexpr year_month_weekday_last& year_month_weekday_last::operator-=(const years& __dy)  noexcept { *this = *this - __dy; return *this; }
+
+#endif // _LIBCPP_STD_VER > 17
 } // chrono
 
 #if _LIBCPP_STD_VER > 11
 // Suffixes for duration literals [time.duration.literals]
 inline namespace literals
-{ 
+{
   inline namespace chrono_literals
   {
 
@@ -1633,7 +2793,7 @@ inline namespace literals
     {
         return chrono::duration<long double, micro> (__us);
     }
-    
+
 
     constexpr chrono::nanoseconds operator""ns(unsigned long long __ns)
     {
@@ -1645,6 +2805,17 @@ inline namespace literals
         return chrono::duration<long double, nano> (__ns);
     }
 
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CXX20_CHRONO_LITERALS)
+    constexpr chrono::day operator ""d(unsigned long long __d) noexcept
+    {
+        return chrono::day(static_cast<unsigned>(__d));
+    }
+ 
+    constexpr chrono::year operator ""y(unsigned long long __y) noexcept
+    {
+        return chrono::year(static_cast<int>(__y));
+    }
+#endif
 }}
 
 namespace chrono { // hoist the literals into namespace std::chrono
@@ -1655,6 +2826,40 @@ namespace chrono { // hoist the literals into namespace std::chrono
 
 _LIBCPP_END_NAMESPACE_STD
 
+#ifndef _LIBCPP_CXX03_LANG
+_LIBCPP_BEGIN_NAMESPACE_FILESYSTEM
+struct _FilesystemClock {
+#if !defined(_LIBCPP_HAS_NO_INT128)
+  typedef __int128_t rep;
+  typedef nano period;
+#else
+  typedef long long rep;
+  typedef nano period;
+#endif
+
+  typedef chrono::duration<rep, period> duration;
+  typedef chrono::time_point<_FilesystemClock> time_point;
+
+  static _LIBCPP_CONSTEXPR_AFTER_CXX11 const bool is_steady = false;
+
+  _LIBCPP_FUNC_VIS static time_point now() noexcept;
+
+  _LIBCPP_INLINE_VISIBILITY
+  static time_t to_time_t(const time_point& __t) noexcept {
+      typedef chrono::duration<rep> __secs;
+      return time_t(
+          chrono::duration_cast<__secs>(__t.time_since_epoch()).count());
+  }
+
+  _LIBCPP_INLINE_VISIBILITY
+  static time_point from_time_t(time_t __t) noexcept {
+      typedef chrono::duration<rep> __secs;
+      return time_point(__secs(__t));
+  }
+};
+_LIBCPP_END_NAMESPACE_FILESYSTEM
+#endif // !_LIBCPP_CXX03_LANG
+
 _LIBCPP_POP_MACROS
 
 #endif  // _LIBCPP_CHRONO
diff --git a/include/cmath b/include/cmath
index ffb1c46c7b68..f5f62adcfb8d 100644
--- a/include/cmath
+++ b/include/cmath
@@ -303,6 +303,7 @@ long double    truncl(long double x);
 
 #include <__config>
 #include <math.h>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
diff --git a/include/complex b/include/complex
index d692ee319204..8cf6a946d711 100644
--- a/include/complex
+++ b/include/complex
@@ -245,6 +245,7 @@ template<class T, class charT, class traits>
 #include <stdexcept>
 #include <cmath>
 #include <sstream>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
diff --git a/include/cstddef b/include/cstddef
index adeefdac9be6..b4c42b19ddb2 100644
--- a/include/cstddef
+++ b/include/cstddef
@@ -35,6 +35,7 @@ Types:
 */
 
 #include <__config>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
@@ -50,7 +51,7 @@ using ::ptrdiff_t;
 using ::size_t;
 
 #if defined(__CLANG_MAX_ALIGN_T_DEFINED) || defined(_GCC_MAX_ALIGN_T) || \
-    defined(__DEFINED_max_align_t)
+    defined(__DEFINED_max_align_t) || defined(__NetBSD__)
 // Re-use the compiler's <stddef.h> max_align_t where possible.
 using ::max_align_t;
 #else
@@ -66,10 +67,10 @@ enum class byte : unsigned char {};
 
 constexpr byte  operator| (byte  __lhs, byte __rhs) noexcept
 {
-	return static_cast<byte>(
-	  static_cast<unsigned char>(
-	     static_cast<unsigned int>(__lhs) | static_cast<unsigned int>(__rhs)
-	));
+    return static_cast<byte>(
+      static_cast<unsigned char>(
+         static_cast<unsigned int>(__lhs) | static_cast<unsigned int>(__rhs)
+    ));
 }
 
 constexpr byte& operator|=(byte& __lhs, byte __rhs) noexcept
@@ -77,10 +78,10 @@ constexpr byte& operator|=(byte& __lhs, byte __rhs) noexcept
 
 constexpr byte  operator& (byte  __lhs, byte __rhs) noexcept
 {
-	return static_cast<byte>(
-	  static_cast<unsigned char>(
-	     static_cast<unsigned int>(__lhs) & static_cast<unsigned int>(__rhs)
-	));
+    return static_cast<byte>(
+      static_cast<unsigned char>(
+         static_cast<unsigned int>(__lhs) & static_cast<unsigned int>(__rhs)
+    ));
 }
 
 constexpr byte& operator&=(byte& __lhs, byte __rhs) noexcept
@@ -88,13 +89,13 @@ constexpr byte& operator&=(byte& __lhs, byte __rhs) noexcept
 
 constexpr byte  operator^ (byte  __lhs, byte __rhs) noexcept
 {
-	return static_cast<byte>(
-	  static_cast<unsigned char>(
-	     static_cast<unsigned int>(__lhs) ^ static_cast<unsigned int>(__rhs)
-	));
+    return static_cast<byte>(
+      static_cast<unsigned char>(
+         static_cast<unsigned int>(__lhs) ^ static_cast<unsigned int>(__rhs)
+    ));
 }
 
-constexpr byte& operator^=(byte& __lhs, byte __rhs) noexcept 
+constexpr byte& operator^=(byte& __lhs, byte __rhs) noexcept
 { return __lhs = __lhs ^ __rhs; }
 
 constexpr byte  operator~ (byte __b) noexcept
diff --git a/include/cstdlib b/include/cstdlib
index 78c428403c37..00c604e67623 100644
--- a/include/cstdlib
+++ b/include/cstdlib
@@ -151,11 +151,11 @@ using ::mbtowc;
 using ::wctomb;
 using ::mbstowcs;
 using ::wcstombs;
-#ifdef _LIBCPP_HAS_QUICK_EXIT
+#if !defined(_LIBCPP_CXX03_LANG) && defined(_LIBCPP_HAS_QUICK_EXIT)
 using ::at_quick_exit;
 using ::quick_exit;
 #endif
-#ifdef _LIBCPP_HAS_C11_FEATURES
+#if _LIBCPP_STD_VER > 14 && defined(_LIBCPP_HAS_C11_FEATURES)
 using ::aligned_alloc;
 #endif
 
diff --git a/include/ctime b/include/ctime
index 81cf11a466c2..8264fe33b964 100644
--- a/include/ctime
+++ b/include/ctime
@@ -73,7 +73,7 @@ using ::gmtime;
 using ::localtime;
 #endif
 using ::strftime;
-#if _LIBCPP_STD_VER > 14 && defined(_LIBCPP_HAS_C11_FEATURES)
+#if _LIBCPP_STD_VER > 14 && defined(_LIBCPP_HAS_TIMESPEC_GET)
 using ::timespec_get;
 #endif
 
diff --git a/include/deque b/include/deque
index bfbd3a5ef543..6f7d04be52be 100644
--- a/include/deque
+++ b/include/deque
@@ -150,6 +150,11 @@ template <class T, class Allocator>
     void swap(deque<T,Allocator>& x, deque<T,Allocator>& y)
          noexcept(noexcept(x.swap(y)));
 
+template <class T, class Allocator, class U>
+    void erase(deque<T, Allocator>& c, const U& value);       // C++20
+template <class T, class Allocator, class Predicate>
+    void erase_if(deque<T, Allocator>& c, Predicate pred);    // C++20
+
 }  // std
 
 */
@@ -161,6 +166,7 @@ template <class T, class Allocator>
 #include <iterator>
 #include <algorithm>
 #include <stdexcept>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
@@ -986,7 +992,7 @@ public:
 #if _LIBCPP_STD_VER >= 14
         _NOEXCEPT;
 #else
-        _NOEXCEPT_(!__alloc_traits::propagate_on_container_swap::value || 
+        _NOEXCEPT_(!__alloc_traits::propagate_on_container_swap::value ||
                     __is_nothrow_swappable<allocator_type>::value);
 #endif
 protected:
@@ -1155,7 +1161,7 @@ __deque_base<_Tp, _Allocator>::swap(__deque_base& __c)
 #if _LIBCPP_STD_VER >= 14
         _NOEXCEPT
 #else
-        _NOEXCEPT_(!__alloc_traits::propagate_on_container_swap::value || 
+        _NOEXCEPT_(!__alloc_traits::propagate_on_container_swap::value ||
                     __is_nothrow_swappable<allocator_type>::value)
 #endif
 {
@@ -2341,7 +2347,7 @@ deque<_Tp, _Allocator>::__add_front_capacity()
                 _Dp(__a, __base::__block_size));
         __buf.push_back(__hold.get());
         __hold.release();
-    
+
         for (typename __base::__map_pointer __i = __base::__map_.begin();
                 __i != __base::__map_.end(); ++__i)
             __buf.push_back(*__i);
@@ -2603,6 +2609,7 @@ template <class _Tp, class _Allocator>
 void
 deque<_Tp, _Allocator>::pop_back()
 {
+    _LIBCPP_ASSERT(!empty(), "deque::pop_back called for empty deque");
     allocator_type& __a = __base::__alloc();
     size_type __p = __base::size() + __base::__start_ - 1;
     __alloc_traits::destroy(__a, __to_raw_pointer(*(__base::__map_.begin() +
@@ -2853,7 +2860,7 @@ deque<_Tp, _Allocator>::swap(deque& __c)
 #if _LIBCPP_STD_VER >= 14
         _NOEXCEPT
 #else
-        _NOEXCEPT_(!__alloc_traits::propagate_on_container_swap::value || 
+        _NOEXCEPT_(!__alloc_traits::propagate_on_container_swap::value ||
                     __is_nothrow_swappable<allocator_type>::value)
 #endif
 {
@@ -2926,6 +2933,19 @@ swap(deque<_Tp, _Allocator>& __x, deque<_Tp, _Allocator>& __y)
     __x.swap(__y);
 }
 
+#if _LIBCPP_STD_VER > 17
+template <class _Tp, class _Allocator, class _Up>
+inline _LIBCPP_INLINE_VISIBILITY
+void erase(deque<_Tp, _Allocator>& __c, const _Up& __v)
+{ __c.erase(_VSTD::remove(__c.begin(), __c.end(), __v), __c.end()); }
+
+template <class _Tp, class _Allocator, class _Predicate>
+inline _LIBCPP_INLINE_VISIBILITY
+void erase_if(deque<_Tp, _Allocator>& __c, _Predicate __pred)
+{ __c.erase(_VSTD::remove_if(__c.begin(), __c.end(), __pred), __c.end()); }
+#endif
+
+
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
diff --git a/include/exception b/include/exception
index b517486b5a8e..fdd83d10c3da 100644
--- a/include/exception
+++ b/include/exception
@@ -81,6 +81,7 @@ template <class E> void rethrow_if_nested(const E& e);
 #include <cstddef>
 #include <cstdlib>
 #include <type_traits>
+#include <version>
 
 #if defined(_LIBCPP_ABI_MICROSOFT) && !defined(_LIBCPP_NO_VCRUNTIME)
 #include <vcruntime_exception.h>
@@ -163,7 +164,7 @@ public:
 };
 
 template<class _Ep>
-exception_ptr
+_LIBCPP_INLINE_VISIBILITY exception_ptr
 make_exception_ptr(_Ep __e) _NOEXCEPT
 {
 #ifndef _LIBCPP_NO_EXCEPTIONS
@@ -222,7 +223,7 @@ _LIBCPP_NORETURN _LIBCPP_FUNC_VIS void rethrow_exception(exception_ptr p);
 template <class _E> void *__GetExceptionInfo(_E);
 
 template<class _Ep>
-exception_ptr
+_LIBCPP_INLINE_VISIBILITY exception_ptr
 make_exception_ptr(_Ep __e) _NOEXCEPT
 {
   return __copy_exception_ptr(_VSTD::addressof(__e), __GetExceptionInfo(__e));
diff --git a/include/experimental/any b/include/experimental/any
index 1dcdd0f25ec3..d9c95342589f 100644
--- a/include/experimental/any
+++ b/include/experimental/any
@@ -1,11 +1,21 @@
 // -*- C++ -*-
-//===------------------------------ any -----------------------------------===//
+//===------------------------------- any ----------------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
-// This file is distributed under the University of Illinois Open Source
-// License. See LICENSE.TXT for details.
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
+#ifndef _LIBCPP_EXPERIMENTAL_ANY
+#define _LIBCPP_EXPERIMENTAL_ANY
 
-#error "<experimental/any> has been removed. Use <any> instead."
+#include <__config>
+
+#ifdef _LIBCPP_WARNING
+_LIBCPP_WARNING("<experimental/any> has been removed. Use <any> instead.")
+#else
+# warning "<experimental/any> has been removed. Use <any> instead."
+#endif
+
+#endif // _LIBCPP_EXPERIMENTAL_ANY
diff --git a/include/experimental/chrono b/include/experimental/chrono
index 591cf7160c13..30c7e4a9d5ac 100644
--- a/include/experimental/chrono
+++ b/include/experimental/chrono
@@ -1,11 +1,21 @@
 // -*- C++ -*-
-//===------------------------------ chrono ---------------------------------===//
+//===---------------------------- chrono ----------------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
-// This file is distributed under the University of Illinois Open Source
-// License. See LICENSE.TXT for details.
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
+#ifndef _LIBCPP_EXPERIMENTAL_CHRONO
+#define _LIBCPP_EXPERIMENTAL_CHRONO
 
-#error "<experimental/chrono> has been removed. Use <chrono> instead."
+#include <__config>
+
+#ifdef _LIBCPP_WARNING
+_LIBCPP_WARNING("<experimental/chrono> has been removed. Use <chrono> instead.")
+#else
+# warning "<experimental/chrono> has been removed. Use <chrono> instead."
+#endif
+
+#endif // _LIBCPP_EXPERIMENTAL_CHRONO
diff --git a/include/experimental/coroutine b/include/experimental/coroutine
index 1eb224a535a1..7cb39b81b48f 100644
--- a/include/experimental/coroutine
+++ b/include/experimental/coroutine
@@ -214,7 +214,7 @@ public:
     _LIBCPP_INLINE_VISIBILITY
     _Promise& promise() const {
         return *static_cast<_Promise*>(
-            __builtin_coro_promise(this->__handle_, __alignof(_Promise), false));
+            __builtin_coro_promise(this->__handle_, _LIBCPP_ALIGNOF(_Promise), false));
     }
 
 public:
@@ -254,7 +254,7 @@ public:
         coroutine_handle __tmp;
         __tmp.__handle_ = __builtin_coro_promise(
             _VSTD::addressof(const_cast<_RawPromise&>(__promise)),
-             __alignof(_Promise), true);
+             _LIBCPP_ALIGNOF(_Promise), true);
         return __tmp;
     }
 };
@@ -272,7 +272,7 @@ public:
   _LIBCPP_INLINE_VISIBILITY
   _Promise& promise() const {
     return *static_cast<_Promise*>(
-      __builtin_coro_promise(this->__handle_, __alignof(_Promise), false));
+      __builtin_coro_promise(this->__handle_, _LIBCPP_ALIGNOF(_Promise), false));
   }
 
   _LIBCPP_CONSTEXPR explicit operator bool() const _NOEXCEPT { return true; }
diff --git a/include/experimental/dynarray b/include/experimental/dynarray
deleted file mode 100644
index a60c87c3f970..000000000000
--- a/include/experimental/dynarray
+++ /dev/null
@@ -1,305 +0,0 @@
-// -*- C++ -*-
-//===-------------------------- dynarray ----------------------------------===//
-//
-//                     The LLVM Compiler Infrastructure
-//
-// This file is dual licensed under the MIT and the University of Illinois Open
-// Source Licenses. See LICENSE.TXT for details.
-//
-//===----------------------------------------------------------------------===//
-
-#ifndef _LIBCPP_DYNARRAY
-#define _LIBCPP_DYNARRAY
-
-/*
-    dynarray synopsis
-
-namespace std { namespace experimental {
-
-template< typename T >
-class dynarray
-{
-    // types:
-    typedef       T                               value_type;
-    typedef       T&                              reference;
-    typedef const T&                              const_reference;
-    typedef       T*                              pointer;
-    typedef const T*                              const_pointer;
-    typedef       implementation-defined          iterator;
-    typedef       implementation-defined          const_iterator;
-    typedef reverse_iterator<iterator>            reverse_iterator;
-    typedef reverse_iterator<const_iterator>      const_reverse_iterator;
-    typedef size_t                                size_type;
-    typedef ptrdiff_t                             difference_type;
-
-public:
-    // construct/copy/destroy:
-    explicit dynarray(size_type c);
-    dynarray(size_type c, const T& v);
-    dynarray(const dynarray& d);
-    dynarray(initializer_list<T>);
-
-    template <class Alloc>
-      dynarray(allocator_arg_t, const Alloc& a, size_type c, const Alloc& alloc);
-    template <class Alloc>
-      dynarray(allocator_arg_t, const Alloc& a, size_type c, const T& v, const Alloc& alloc);
-    template <class Alloc>
-      dynarray(allocator_arg_t, const Alloc& a, const dynarray& d, const Alloc& alloc);
-    template <class Alloc>
-      dynarray(allocator_arg_t, const Alloc& a, initializer_list<T>, const Alloc& alloc);
-    dynarray& operator=(const dynarray&) = delete;
-    ~dynarray();
-
-    // iterators:
-    iterator       begin()        noexcept;
-    const_iterator begin()  const noexcept;
-    const_iterator cbegin() const noexcept;
-    iterator       end()          noexcept;
-    const_iterator end()    const noexcept;
-    const_iterator cend()   const noexcept;
-
-    reverse_iterator       rbegin()        noexcept;
-    const_reverse_iterator rbegin()  const noexcept;
-    const_reverse_iterator crbegin() const noexcept;
-    reverse_iterator       rend()          noexcept;
-    const_reverse_iterator rend()    const noexcept;
-    const_reverse_iterator crend()   const noexcept;
-
-    // capacity:
-    size_type size()     const noexcept;
-    size_type max_size() const noexcept;
-    bool      empty()    const noexcept;
-
-    // element access:
-    reference       operator[](size_type n);
-    const_reference operator[](size_type n) const;
-
-    reference       front();
-    const_reference front() const;
-    reference       back();
-    const_reference back()  const;
-
-    const_reference at(size_type n) const;
-    reference       at(size_type n);
-
-    // data access:
-    T*       data()       noexcept;
-    const T* data() const noexcept;
-
-    // mutating member functions:
-    void fill(const T& v);
-};
-
-}}  // std::experimental
-
-*/
-#include <__config>
-#if _LIBCPP_STD_VER > 11
-
-#include <__functional_base>
-#include <iterator>
-#include <stdexcept>
-#include <initializer_list>
-#include <new>
-#include <algorithm>
-
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#pragma GCC system_header
-#endif
-
-_LIBCPP_PUSH_MACROS
-#include <__undef_macros>
-
-namespace std { namespace experimental { inline namespace __array_extensions_v1 {
-
-template <class _Tp>
-struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_DYNARRAY dynarray
-{
-public:
-    // types:
-    typedef dynarray __self;
-    typedef _Tp                                   value_type;
-    typedef value_type&                           reference;
-    typedef const value_type&                     const_reference;
-    typedef value_type*                           iterator;
-    typedef const value_type*                     const_iterator;
-    typedef value_type*                           pointer;
-    typedef const value_type*                     const_pointer;
-    typedef size_t                                size_type;
-    typedef ptrdiff_t                             difference_type;
-    typedef std::reverse_iterator<iterator>       reverse_iterator;
-    typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
-
-private:
-    size_t                  __size_;
-    value_type *            __base_;
-    _LIBCPP_INLINE_VISIBILITY dynarray () noexcept :  __size_(0), __base_(nullptr) {}
-    
-    static inline _LIBCPP_INLINE_VISIBILITY
-    value_type* __allocate(size_t __count) {
-        if (numeric_limits<size_t>::max() / sizeof (value_type) <= __count)
-            __throw_bad_array_length();
-
-        return static_cast<value_type *>(
-            _VSTD::__libcpp_allocate(sizeof(value_type) * __count, __alignof(value_type)));
-    }
-
-    static inline _LIBCPP_INLINE_VISIBILITY
-    void __deallocate_value(value_type* __ptr ) noexcept {
-        _VSTD::__libcpp_deallocate(static_cast<void *>(__ptr), __alignof(value_type));
-    }
-
-public:
-
-    _LIBCPP_INLINE_VISIBILITY
-    explicit dynarray(size_type __c);
-    _LIBCPP_INLINE_VISIBILITY
-    dynarray(size_type __c, const value_type& __v);
-    _LIBCPP_INLINE_VISIBILITY
-    dynarray(const dynarray& __d);
-    _LIBCPP_INLINE_VISIBILITY
-    dynarray(initializer_list<value_type>);
-
-//  We're not implementing these right now.
-//  Updated with the resolution of LWG issue #2255
-//     template <typename _Alloc>
-//       dynarray(allocator_arg_t, const _Alloc& __alloc, size_type __c);
-//     template <typename _Alloc>
-//       dynarray(allocator_arg_t, const _Alloc& __alloc, size_type __c, const value_type& __v);
-//     template <typename _Alloc>
-//       dynarray(allocator_arg_t, const _Alloc& __alloc, const dynarray& __d);
-//     template <typename _Alloc>
-//       dynarray(allocator_arg_t, const _Alloc& __alloc, initializer_list<value_type>);
-
-    dynarray& operator=(const dynarray&) = delete;
-    _LIBCPP_INLINE_VISIBILITY
-    ~dynarray();
-
-    // iterators:
-    inline _LIBCPP_INLINE_VISIBILITY iterator       begin()        noexcept { return iterator(data()); }
-    inline _LIBCPP_INLINE_VISIBILITY const_iterator begin()  const noexcept { return const_iterator(data()); }
-    inline _LIBCPP_INLINE_VISIBILITY const_iterator cbegin() const noexcept { return const_iterator(data()); }
-    inline _LIBCPP_INLINE_VISIBILITY iterator       end()          noexcept { return iterator(data() + __size_); }
-    inline _LIBCPP_INLINE_VISIBILITY const_iterator end()    const noexcept { return const_iterator(data() + __size_); }
-    inline _LIBCPP_INLINE_VISIBILITY const_iterator cend()   const noexcept { return const_iterator(data() + __size_); }
-
-    inline _LIBCPP_INLINE_VISIBILITY reverse_iterator       rbegin()        noexcept { return reverse_iterator(end()); }
-    inline _LIBCPP_INLINE_VISIBILITY const_reverse_iterator rbegin()  const noexcept { return const_reverse_iterator(end()); }
-    inline _LIBCPP_INLINE_VISIBILITY const_reverse_iterator crbegin() const noexcept { return const_reverse_iterator(end()); }
-    inline _LIBCPP_INLINE_VISIBILITY reverse_iterator       rend()          noexcept { return reverse_iterator(begin()); }
-    inline _LIBCPP_INLINE_VISIBILITY const_reverse_iterator rend()    const noexcept { return const_reverse_iterator(begin()); }
-    inline _LIBCPP_INLINE_VISIBILITY const_reverse_iterator crend()   const noexcept { return const_reverse_iterator(begin()); }
-
-    // capacity:
-    inline _LIBCPP_INLINE_VISIBILITY size_type size()     const noexcept { return __size_; }
-    inline _LIBCPP_INLINE_VISIBILITY size_type max_size() const noexcept { return __size_; }
-    inline _LIBCPP_INLINE_VISIBILITY bool      empty()    const noexcept { return __size_ == 0; }
-
-    // element access:
-    inline _LIBCPP_INLINE_VISIBILITY reference       operator[](size_type __n)       { return data()[__n]; }
-    inline _LIBCPP_INLINE_VISIBILITY const_reference operator[](size_type __n) const { return data()[__n]; }
-
-    inline _LIBCPP_INLINE_VISIBILITY reference       front()       { return data()[0]; }
-    inline _LIBCPP_INLINE_VISIBILITY const_reference front() const { return data()[0]; }
-    inline _LIBCPP_INLINE_VISIBILITY reference       back()        { return data()[__size_-1]; }
-    inline _LIBCPP_INLINE_VISIBILITY const_reference back()  const { return data()[__size_-1]; }
-
-    inline _LIBCPP_INLINE_VISIBILITY const_reference at(size_type __n) const;
-    inline _LIBCPP_INLINE_VISIBILITY reference       at(size_type __n);
-
-    // data access:
-    inline _LIBCPP_INLINE_VISIBILITY _Tp*       data()       noexcept { return __base_; }
-    inline _LIBCPP_INLINE_VISIBILITY const _Tp* data() const noexcept { return __base_; }
-
-    // mutating member functions:
-    inline _LIBCPP_INLINE_VISIBILITY void fill(const value_type& __v) { fill_n(begin(), __size_, __v); }
-};
-
-template <class _Tp>
-inline
-dynarray<_Tp>::dynarray(size_type __c) : dynarray ()
-{
-    __base_ = __allocate (__c);
-    value_type *__data = data ();
-    for ( __size_ = 0; __size_ < __c; ++__size_, ++__data )
-        ::new (__data) value_type;
-}
-
-template <class _Tp>
-inline
-dynarray<_Tp>::dynarray(size_type __c, const value_type& __v) : dynarray ()
-{
-    __base_ = __allocate (__c);
-    value_type *__data = data ();
-    for ( __size_ = 0; __size_ < __c; ++__size_, ++__data )
-        ::new (__data) value_type (__v);
-}
-
-template <class _Tp>
-inline
-dynarray<_Tp>::dynarray(initializer_list<value_type> __il) : dynarray ()
-{
-    size_t sz = __il.size();
-    __base_ = __allocate (sz);
-    value_type *__data = data ();
-    auto src = __il.begin();
-    for ( __size_ = 0; __size_ < sz; ++__size_, ++__data, ++src )
-        ::new (__data) value_type (*src);
-}
-
-template <class _Tp>
-inline
-dynarray<_Tp>::dynarray(const dynarray& __d) : dynarray ()
-{
-    size_t sz = __d.size();
-    __base_ = __allocate (sz);
-    value_type *__data = data ();
-    auto src = __d.begin();
-    for ( __size_ = 0; __size_ < sz; ++__size_, ++__data, ++src )
-        ::new (__data) value_type (*src);
-}
-
-template <class _Tp>
-inline
-dynarray<_Tp>::~dynarray()
-{ 
-    value_type *__data = data () + __size_;
-    for ( size_t i = 0; i < __size_; ++i )
-        (--__data)->value_type::~value_type();
-    __deallocate_value( __base_ );
-}
-
-template <class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY
-typename dynarray<_Tp>::reference
-dynarray<_Tp>::at(size_type __n)
-{
-    if (__n >= __size_)
-        __throw_out_of_range("dynarray::at");
-
-    return data()[__n];
-}
-
-template <class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY
-typename dynarray<_Tp>::const_reference
-dynarray<_Tp>::at(size_type __n) const
-{
-    if (__n >= __size_)
-        __throw_out_of_range("dynarray::at");
-
-    return data()[__n];
-}
-
-}}}
-
-
-_LIBCPP_BEGIN_NAMESPACE_STD
-template <class _Tp, class _Alloc>
-struct _LIBCPP_TEMPLATE_VIS uses_allocator<std::experimental::dynarray<_Tp>, _Alloc> : true_type {};
-_LIBCPP_END_NAMESPACE_STD
-
-_LIBCPP_POP_MACROS
-
-#endif  // if _LIBCPP_STD_VER > 11 
-#endif  // _LIBCPP_DYNARRAY
diff --git a/include/experimental/memory_resource b/include/experimental/memory_resource
index 221ce5b8eacc..83781d462034 100644
--- a/include/experimental/memory_resource
+++ b/include/experimental/memory_resource
@@ -98,7 +98,7 @@ size_t __aligned_allocation_size(size_t __s, size_t __a) _NOEXCEPT
 // 8.5, memory.resource
 class _LIBCPP_TYPE_VIS memory_resource
 {
-    static const size_t __max_align = alignof(max_align_t);
+    static const size_t __max_align = _LIBCPP_ALIGNOF(max_align_t);
 
 // 8.5.2, memory.resource.public
 public:
@@ -190,7 +190,7 @@ public:
                 " 'n' exceeds maximum supported size");
         }
         return static_cast<_ValueType*>(
-            __res_->allocate(__n * sizeof(_ValueType), alignof(_ValueType))
+            __res_->allocate(__n * sizeof(_ValueType), _LIBCPP_ALIGNOF(_ValueType))
         );
     }
 
@@ -198,7 +198,7 @@ public:
     void deallocate(_ValueType * __p, size_t __n) _NOEXCEPT {
         _LIBCPP_ASSERT(__n <= __max_size(),
                        "deallocate called for size which exceeds max_size()");
-        __res_->deallocate(__p, __n * sizeof(_ValueType), alignof(_ValueType));
+        __res_->deallocate(__p, __n * sizeof(_ValueType), _LIBCPP_ALIGNOF(_ValueType));
     }
 
     template <class _Tp, class ..._Ts>
@@ -345,7 +345,7 @@ class _LIBCPP_TEMPLATE_VIS __resource_adaptor_imp
                && is_same<typename _CTraits::pointer, char*>::value
                && is_same<typename _CTraits::void_pointer, void*>::value, "");
 
-    static const size_t _MaxAlign = alignof(max_align_t);
+    static const size_t _MaxAlign = _LIBCPP_ALIGNOF(max_align_t);
 
     using _Alloc = typename _CTraits::template rebind_alloc<
             typename aligned_storage<_MaxAlign, _MaxAlign>::type
diff --git a/include/experimental/numeric b/include/experimental/numeric
index 14a664011b56..19c65313f0f2 100644
--- a/include/experimental/numeric
+++ b/include/experimental/numeric
@@ -7,5 +7,15 @@
 // Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
+#ifndef _LIBCPP_EXPERIMENTAL_NUMERIC
+#define _LIBCPP_EXPERIMENTAL_NUMERIC
 
-#error "<experimental/numeric> has been removed. Use <numeric> instead."
+#include <__config>
+
+#ifdef _LIBCPP_WARNING
+_LIBCPP_WARNING("<experimental/numeric> has been removed. Use <numeric> instead.")
+#else
+# warning "<experimental/numeric> has been removed. Use <numeric> instead."
+#endif
+
+#endif // _LIBCPP_EXPERIMENTAL_NUMERIC
diff --git a/include/experimental/optional b/include/experimental/optional
index d68cefdf6c1d..6eb4a2618d2c 100644
--- a/include/experimental/optional
+++ b/include/experimental/optional
@@ -7,5 +7,15 @@
 // Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
+#ifndef _LIBCPP_EXPERIMENTAL_OPTIONAL
+#define _LIBCPP_EXPERIMENTAL_OPTIONAL
 
-#error "<experimental/optional> has been removed. Use <optional> instead."
+#include <__config>
+
+#ifdef _LIBCPP_WARNING
+_LIBCPP_WARNING("<experimental/optional> has been removed. Use <optional> instead.")
+#else
+# warning "<experimental/optional> has been removed. Use <optional> instead."
+#endif
+
+#endif // _LIBCPP_EXPERIMENTAL_OPTIONAL
diff --git a/include/experimental/ratio b/include/experimental/ratio
index 9c2bf2e4624e..52c12004dba0 100644
--- a/include/experimental/ratio
+++ b/include/experimental/ratio
@@ -1,11 +1,21 @@
 // -*- C++ -*-
-//===------------------------------ ratio ---------------------------------===//
+//===----------------------------- ratio ----------------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
-// This file is distributed under the University of Illinois Open Source
-// License. See LICENSE.TXT for details.
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
+#ifndef _LIBCPP_EXPERIMENTAL_RATIO
+#define _LIBCPP_EXPERIMENTAL_RATIO
 
-#error "<experimental/ratio> has been removed. Use <ratio> instead."
+#include <__config>
+
+#ifdef _LIBCPP_WARNING
+_LIBCPP_WARNING("<experimental/ratio> has been removed. Use <ratio> instead.")
+#else
+# warning "<experimental/ratio> has been removed. Use <ratio> instead."
+#endif
+
+#endif // _LIBCPP_EXPERIMENTAL_RATIO
diff --git a/include/experimental/string_view b/include/experimental/string_view
index f13bff54d531..100bdfe72735 100644
--- a/include/experimental/string_view
+++ b/include/experimental/string_view
@@ -3,9 +3,19 @@
 //
 //                     The LLVM Compiler Infrastructure
 //
-// This file is distributed under the University of Illinois Open Source
-// License. See LICENSE.TXT for details.
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
+#ifndef _LIBCPP_EXPERIMENTAL_STRING_VIEW
+#define _LIBCPP_EXPERIMENTAL_STRING_VIEW
 
-#error "<experimental/string_view> has been removed. Use <string_view> instead."
+#include <__config>
+
+#ifdef _LIBCPP_WARNING
+_LIBCPP_WARNING("<experimental/string_view> has been removed. Use <string_view> instead.")
+#else
+# warning "<experimental/string_view> has been removed. Use <string_view> instead."
+#endif
+
+#endif // _LIBCPP_EXPERIMENTAL_STRING_VIEW
diff --git a/include/experimental/system_error b/include/experimental/system_error
index 7937357fa141..1cf84ee01251 100644
--- a/include/experimental/system_error
+++ b/include/experimental/system_error
@@ -7,5 +7,15 @@
 // Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
+#ifndef _LIBCPP_EXPERIMENTAL_SYSTEM_ERROR
+#define _LIBCPP_EXPERIMENTAL_SYSTEM_ERROR
 
-#error "<experimental/system_error> has been removed. Use <system_error> instead."
+#include <__config>
+
+#ifdef _LIBCPP_WARNING
+_LIBCPP_WARNING("<experimental/system_error> has been removed. Use <system_error> instead.")
+#else
+# warning "<experimental/system_error> has been removed. Use <system_error> instead."
+#endif
+
+#endif // _LIBCPP_EXPERIMENTAL_SYSTEM_ERROR
diff --git a/include/experimental/tuple b/include/experimental/tuple
index 1f37a6293bac..6d71bb559b02 100644
--- a/include/experimental/tuple
+++ b/include/experimental/tuple
@@ -7,5 +7,15 @@
 // Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
+#ifndef _LIBCPP_EXPERIMENTAL_TUPLE
+#define _LIBCPP_EXPERIMENTAL_TUPLE
 
-#error "<experimental/tuple> has been removed. Use <tuple> instead."
+#include <__config>
+
+#ifdef _LIBCPP_WARNING
+_LIBCPP_WARNING("<experimental/tuple> has been removed. Use <tuple> instead.")
+#else
+# warning "<experimental/tuple> has been removed. Use <tuple> instead."
+#endif
+
+#endif // _LIBCPP_EXPERIMENTAL_TUPLE
diff --git a/include/filesystem b/include/filesystem
index aa1d71800725..af713a063587 100644
--- a/include/filesystem
+++ b/include/filesystem
@@ -244,6 +244,7 @@
 #include <utility>
 #include <iomanip> // for quoted
 #include <string_view>
+#include <version>
 
 #include <__debug>
 
@@ -256,42 +257,8 @@ _LIBCPP_PUSH_MACROS
 
 #ifndef _LIBCPP_CXX03_LANG
 
-#if _LIBCPP_STD_VER >= 17
-#define __cpp_lib_filesystem 201703
-#endif
-
 _LIBCPP_BEGIN_NAMESPACE_FILESYSTEM
 
-struct _FilesystemClock {
-#if !defined(_LIBCPP_HAS_NO_INT128)
-  typedef __int128_t rep;
-  typedef nano period;
-#else
-  typedef long long rep;
-  typedef nano period;
-#endif
-
-  typedef chrono::duration<rep, period> duration;
-  typedef chrono::time_point<_FilesystemClock> time_point;
-
-  static _LIBCPP_CONSTEXPR_AFTER_CXX11 const bool is_steady = false;
-
-  _LIBCPP_FUNC_VIS static time_point now() noexcept;
-
-  _LIBCPP_INLINE_VISIBILITY
-  static time_t to_time_t(const time_point& __t) noexcept {
-    typedef chrono::duration<rep> __secs;
-    return time_t(
-        chrono::duration_cast<__secs>(__t.time_since_epoch()).count());
-  }
-
-  _LIBCPP_INLINE_VISIBILITY
-  static time_point from_time_t(time_t __t) noexcept {
-    typedef chrono::duration<rep> __secs;
-    return time_point(__secs(__t));
-  }
-};
-
 typedef chrono::time_point<_FilesystemClock> file_time_type;
 
 struct _LIBCPP_TYPE_VIS space_info {
@@ -590,7 +557,7 @@ template <class _ECharT>
 typename enable_if<__can_convert_char<_ECharT>::value, bool>::type
 __is_separator(_ECharT __e) {
   return __e == _ECharT('/');
-};
+}
 
 struct _NullSentinal {};
 
@@ -1184,6 +1151,31 @@ public:
     return __is;
   }
 
+  friend _LIBCPP_INLINE_VISIBILITY bool operator==(const path& __lhs, const path& __rhs) noexcept {
+    return __lhs.compare(__rhs) == 0;
+  }
+  friend _LIBCPP_INLINE_VISIBILITY bool operator!=(const path& __lhs, const path& __rhs) noexcept {
+    return __lhs.compare(__rhs) != 0;
+  }
+  friend _LIBCPP_INLINE_VISIBILITY bool operator<(const path& __lhs, const path& __rhs) noexcept {
+    return __lhs.compare(__rhs) < 0;
+  }
+  friend _LIBCPP_INLINE_VISIBILITY bool operator<=(const path& __lhs, const path& __rhs) noexcept {
+    return __lhs.compare(__rhs) <= 0;
+  }
+  friend _LIBCPP_INLINE_VISIBILITY bool operator>(const path& __lhs, const path& __rhs) noexcept {
+    return __lhs.compare(__rhs) > 0;
+  }
+  friend _LIBCPP_INLINE_VISIBILITY bool operator>=(const path& __lhs, const path& __rhs) noexcept {
+    return __lhs.compare(__rhs) >= 0;
+  }
+
+  friend _LIBCPP_INLINE_VISIBILITY path operator/(const path& __lhs,
+                                                  const path& __rhs) {
+    path __result(__lhs);
+    __result /= __rhs;
+    return __result;
+  }
 private:
   inline _LIBCPP_INLINE_VISIBILITY path&
   __assign_view(__string_view const& __s) noexcept {
@@ -1200,43 +1192,6 @@ inline _LIBCPP_INLINE_VISIBILITY void swap(path& __lhs, path& __rhs) noexcept {
 _LIBCPP_FUNC_VIS
 size_t hash_value(const path& __p) noexcept;
 
-inline _LIBCPP_INLINE_VISIBILITY bool operator==(const path& __lhs,
-                                                 const path& __rhs) noexcept {
-  return __lhs.compare(__rhs) == 0;
-}
-
-inline _LIBCPP_INLINE_VISIBILITY bool operator!=(const path& __lhs,
-                                                 const path& __rhs) noexcept {
-  return __lhs.compare(__rhs) != 0;
-}
-
-inline _LIBCPP_INLINE_VISIBILITY bool operator<(const path& __lhs,
-                                                const path& __rhs) noexcept {
-  return __lhs.compare(__rhs) < 0;
-}
-
-inline _LIBCPP_INLINE_VISIBILITY bool operator<=(const path& __lhs,
-                                                 const path& __rhs) noexcept {
-  return __lhs.compare(__rhs) <= 0;
-}
-
-inline _LIBCPP_INLINE_VISIBILITY bool operator>(const path& __lhs,
-                                                const path& __rhs) noexcept {
-  return __lhs.compare(__rhs) > 0;
-}
-
-inline _LIBCPP_INLINE_VISIBILITY bool operator>=(const path& __lhs,
-                                                 const path& __rhs) noexcept {
-  return __lhs.compare(__rhs) >= 0;
-}
-
-inline _LIBCPP_INLINE_VISIBILITY path operator/(const path& __lhs,
-                                                const path& __rhs) {
-  path __result(__lhs);
-  __result /= __rhs;
-  return __result;
-}
-
 template <class _Source>
 _LIBCPP_INLINE_VISIBILITY
     typename enable_if<__is_pathable<_Source>::value, path>::type
diff --git a/include/forward_list b/include/forward_list
index 571afdc925b0..b506acd1ff20 100644
--- a/include/forward_list
+++ b/include/forward_list
@@ -167,6 +167,11 @@ template <class T, class Allocator>
     void swap(forward_list<T, Allocator>& x, forward_list<T, Allocator>& y)
          noexcept(noexcept(x.swap(y)));
 
+template <class T, class Allocator, class U>
+    void erase(forward_list<T, Allocator>& c, const U& value);       // C++20
+template <class T, class Allocator, class Predicate>
+    void erase_if(forward_list<T, Allocator>& c, Predicate pred);    // C++20
+
 }  // std
 
 */
@@ -177,6 +182,7 @@ template <class T, class Allocator>
 #include <limits>
 #include <iterator>
 #include <algorithm>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
@@ -1743,6 +1749,18 @@ swap(forward_list<_Tp, _Alloc>& __x, forward_list<_Tp, _Alloc>& __y)
     __x.swap(__y);
 }
 
+#if _LIBCPP_STD_VER > 17
+template <class _Tp, class _Allocator, class _Predicate>
+inline _LIBCPP_INLINE_VISIBILITY
+void erase_if(forward_list<_Tp, _Allocator>& __c, _Predicate __pred)
+{ __c.remove_if(__pred); }
+
+template <class _Tp, class _Allocator, class _Up>
+inline _LIBCPP_INLINE_VISIBILITY
+void erase(forward_list<_Tp, _Allocator>& __c, const _Up& __v)
+{ _VSTD::erase_if(__c, [&](auto& __elem) { return __elem == __v; }); }
+#endif
+
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
diff --git a/include/fstream b/include/fstream
index 332b4747c1af..711e484e2dcf 100644
--- a/include/fstream
+++ b/include/fstream
@@ -702,6 +702,7 @@ basic_filebuf<_CharT, _Traits>::close()
             __file_ = 0;
         else
             __rt = 0;
+        setbuf(0, 0);
     }
     return __rt;
 }
diff --git a/include/functional b/include/functional
index 6b70f731e1cc..95491879b0c7 100644
--- a/include/functional
+++ b/include/functional
@@ -68,6 +68,11 @@ template <class T> reference_wrapper<const T> cref(const T& t) noexcept;
 template <class T> void cref(const T&& t) = delete;
 template <class T> reference_wrapper<const T> cref(reference_wrapper<T> t) noexcept;
 
+template <class T> struct unwrap_reference;                                       // since C++20
+template <class T> struct unwrap_ref_decay : unwrap_reference<decay_t<T>> { };    // since C++20
+template <class T> using unwrap_reference_t = typename unwrap_reference<T>::type; // since C++20
+template <class T> using unwrap_ref_decay_t = typename unwrap_ref_decay<T>::type; // since C++20
+
 template <class T> // <class T=void> in C++14
 struct plus : binary_function<T, T, T>
 {
@@ -183,7 +188,7 @@ struct bit_xor : unary_function<T, bool>
 };
 
 template <class Predicate>
-class unary_negate
+class unary_negate // deprecated in C++17
     : public unary_function<typename Predicate::argument_type, bool>
 {
 public:
@@ -191,10 +196,11 @@ public:
     bool operator()(const typename Predicate::argument_type& x) const;
 };
 
-template <class Predicate> unary_negate<Predicate> not1(const Predicate& pred);
+template <class Predicate> // deprecated in C++17
+unary_negate<Predicate> not1(const Predicate& pred);
 
 template <class Predicate>
-class binary_negate
+class binary_negate // deprecated in C++17
     : public binary_function<typename Predicate::first_argument_type,
                              typename Predicate::second_argument_type,
                              bool>
@@ -205,7 +211,8 @@ public:
                     const typename Predicate::second_argument_type& y) const;
 };
 
-template <class Predicate> binary_negate<Predicate> not2(const Predicate& pred);
+template <class Predicate> // deprecated in C++17
+binary_negate<Predicate> not2(const Predicate& pred);
 
 template <class F> unspecified not_fn(F&& f); // C++17
 
@@ -487,6 +494,7 @@ POLICY:  For non-variadic implementations, the number of arguments is limited
 #include <memory>
 #include <tuple>
 #include <utility>
+#include <version>
 
 #include <__functional_base>
 
@@ -980,7 +988,7 @@ struct _LIBCPP_TEMPLATE_VIS bit_not<void>
 #endif
 
 template <class _Predicate>
-class _LIBCPP_TEMPLATE_VIS unary_negate
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_DEPRECATED_IN_CXX17 unary_negate
     : public unary_function<typename _Predicate::argument_type, bool>
 {
     _Predicate __pred_;
@@ -994,19 +1002,19 @@ public:
 };
 
 template <class _Predicate>
-inline _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_DEPRECATED_IN_CXX17 inline _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY
 unary_negate<_Predicate>
 not1(const _Predicate& __pred) {return unary_negate<_Predicate>(__pred);}
 
 template <class _Predicate>
-class _LIBCPP_TEMPLATE_VIS binary_negate
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_DEPRECATED_IN_CXX17 binary_negate
     : public binary_function<typename _Predicate::first_argument_type,
                              typename _Predicate::second_argument_type,
                              bool>
 {
     _Predicate __pred_;
 public:
-    _LIBCPP_INLINE_VISIBILITY explicit _LIBCPP_CONSTEXPR_AFTER_CXX11 
+    _LIBCPP_INLINE_VISIBILITY explicit _LIBCPP_CONSTEXPR_AFTER_CXX11
     binary_negate(const _Predicate& __pred) : __pred_(__pred) {}
 
     _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY
@@ -1016,13 +1024,13 @@ public:
 };
 
 template <class _Predicate>
-inline _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_DEPRECATED_IN_CXX17 inline _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY
 binary_negate<_Predicate>
 not2(const _Predicate& __pred) {return binary_negate<_Predicate>(__pred);}
 
 #if _LIBCPP_STD_VER <= 14 || defined(_LIBCPP_ENABLE_CXX17_REMOVED_BINDERS)
 template <class __Operation>
-class _LIBCPP_TEMPLATE_VIS binder1st
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_DEPRECATED_IN_CXX11 binder1st
     : public unary_function<typename __Operation::second_argument_type,
                             typename __Operation::result_type>
 {
@@ -1042,13 +1050,13 @@ public:
 };
 
 template <class __Operation, class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_DEPRECATED_IN_CXX11 inline _LIBCPP_INLINE_VISIBILITY
 binder1st<__Operation>
 bind1st(const __Operation& __op, const _Tp& __x)
     {return binder1st<__Operation>(__op, __x);}
 
 template <class __Operation>
-class _LIBCPP_TEMPLATE_VIS binder2nd
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_DEPRECATED_IN_CXX11 binder2nd
     : public unary_function<typename __Operation::first_argument_type,
                             typename __Operation::result_type>
 {
@@ -1068,13 +1076,13 @@ public:
 };
 
 template <class __Operation, class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_DEPRECATED_IN_CXX11 inline _LIBCPP_INLINE_VISIBILITY
 binder2nd<__Operation>
 bind2nd(const __Operation& __op, const _Tp& __x)
     {return binder2nd<__Operation>(__op, __x);}
 
 template <class _Arg, class _Result>
-class _LIBCPP_TEMPLATE_VIS pointer_to_unary_function
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_DEPRECATED_IN_CXX11 pointer_to_unary_function
     : public unary_function<_Arg, _Result>
 {
     _Result (*__f_)(_Arg);
@@ -1086,13 +1094,13 @@ public:
 };
 
 template <class _Arg, class _Result>
-inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_DEPRECATED_IN_CXX11 inline _LIBCPP_INLINE_VISIBILITY
 pointer_to_unary_function<_Arg,_Result>
 ptr_fun(_Result (*__f)(_Arg))
     {return pointer_to_unary_function<_Arg,_Result>(__f);}
 
 template <class _Arg1, class _Arg2, class _Result>
-class _LIBCPP_TEMPLATE_VIS pointer_to_binary_function
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_DEPRECATED_IN_CXX11 pointer_to_binary_function
     : public binary_function<_Arg1, _Arg2, _Result>
 {
     _Result (*__f_)(_Arg1, _Arg2);
@@ -1104,13 +1112,14 @@ public:
 };
 
 template <class _Arg1, class _Arg2, class _Result>
-inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_DEPRECATED_IN_CXX11 inline _LIBCPP_INLINE_VISIBILITY
 pointer_to_binary_function<_Arg1,_Arg2,_Result>
 ptr_fun(_Result (*__f)(_Arg1,_Arg2))
     {return pointer_to_binary_function<_Arg1,_Arg2,_Result>(__f);}
 
 template<class _Sp, class _Tp>
-class _LIBCPP_TEMPLATE_VIS mem_fun_t : public unary_function<_Tp*, _Sp>
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_DEPRECATED_IN_CXX11 mem_fun_t
+    : public unary_function<_Tp*, _Sp>
 {
     _Sp (_Tp::*__p_)();
 public:
@@ -1121,7 +1130,8 @@ public:
 };
 
 template<class _Sp, class _Tp, class _Ap>
-class _LIBCPP_TEMPLATE_VIS mem_fun1_t : public binary_function<_Tp*, _Ap, _Sp>
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_DEPRECATED_IN_CXX11 mem_fun1_t
+    : public binary_function<_Tp*, _Ap, _Sp>
 {
     _Sp (_Tp::*__p_)(_Ap);
 public:
@@ -1132,19 +1142,20 @@ public:
 };
 
 template<class _Sp, class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_DEPRECATED_IN_CXX11 inline _LIBCPP_INLINE_VISIBILITY
 mem_fun_t<_Sp,_Tp>
 mem_fun(_Sp (_Tp::*__f)())
     {return mem_fun_t<_Sp,_Tp>(__f);}
 
 template<class _Sp, class _Tp, class _Ap>
-inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_DEPRECATED_IN_CXX11 inline _LIBCPP_INLINE_VISIBILITY
 mem_fun1_t<_Sp,_Tp,_Ap>
 mem_fun(_Sp (_Tp::*__f)(_Ap))
     {return mem_fun1_t<_Sp,_Tp,_Ap>(__f);}
 
 template<class _Sp, class _Tp>
-class _LIBCPP_TEMPLATE_VIS mem_fun_ref_t : public unary_function<_Tp, _Sp>
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_DEPRECATED_IN_CXX11 mem_fun_ref_t
+    : public unary_function<_Tp, _Sp>
 {
     _Sp (_Tp::*__p_)();
 public:
@@ -1155,7 +1166,8 @@ public:
 };
 
 template<class _Sp, class _Tp, class _Ap>
-class _LIBCPP_TEMPLATE_VIS mem_fun1_ref_t : public binary_function<_Tp, _Ap, _Sp>
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_DEPRECATED_IN_CXX11 mem_fun1_ref_t
+    : public binary_function<_Tp, _Ap, _Sp>
 {
     _Sp (_Tp::*__p_)(_Ap);
 public:
@@ -1166,19 +1178,20 @@ public:
 };
 
 template<class _Sp, class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_DEPRECATED_IN_CXX11 inline _LIBCPP_INLINE_VISIBILITY
 mem_fun_ref_t<_Sp,_Tp>
 mem_fun_ref(_Sp (_Tp::*__f)())
     {return mem_fun_ref_t<_Sp,_Tp>(__f);}
 
 template<class _Sp, class _Tp, class _Ap>
-inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_DEPRECATED_IN_CXX11 inline _LIBCPP_INLINE_VISIBILITY
 mem_fun1_ref_t<_Sp,_Tp,_Ap>
 mem_fun_ref(_Sp (_Tp::*__f)(_Ap))
     {return mem_fun1_ref_t<_Sp,_Tp,_Ap>(__f);}
 
 template <class _Sp, class _Tp>
-class _LIBCPP_TEMPLATE_VIS const_mem_fun_t : public unary_function<const _Tp*, _Sp>
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_DEPRECATED_IN_CXX11 const_mem_fun_t
+    : public unary_function<const _Tp*, _Sp>
 {
     _Sp (_Tp::*__p_)() const;
 public:
@@ -1189,7 +1202,8 @@ public:
 };
 
 template <class _Sp, class _Tp, class _Ap>
-class _LIBCPP_TEMPLATE_VIS const_mem_fun1_t : public binary_function<const _Tp*, _Ap, _Sp>
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_DEPRECATED_IN_CXX11 const_mem_fun1_t
+    : public binary_function<const _Tp*, _Ap, _Sp>
 {
     _Sp (_Tp::*__p_)(_Ap) const;
 public:
@@ -1200,19 +1214,20 @@ public:
 };
 
 template <class _Sp, class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_DEPRECATED_IN_CXX11 inline _LIBCPP_INLINE_VISIBILITY
 const_mem_fun_t<_Sp,_Tp>
 mem_fun(_Sp (_Tp::*__f)() const)
     {return const_mem_fun_t<_Sp,_Tp>(__f);}
 
 template <class _Sp, class _Tp, class _Ap>
-inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_DEPRECATED_IN_CXX11 inline _LIBCPP_INLINE_VISIBILITY
 const_mem_fun1_t<_Sp,_Tp,_Ap>
 mem_fun(_Sp (_Tp::*__f)(_Ap) const)
     {return const_mem_fun1_t<_Sp,_Tp,_Ap>(__f);}
 
 template <class _Sp, class _Tp>
-class _LIBCPP_TEMPLATE_VIS const_mem_fun_ref_t : public unary_function<_Tp, _Sp>
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_DEPRECATED_IN_CXX11 const_mem_fun_ref_t
+    : public unary_function<_Tp, _Sp>
 {
     _Sp (_Tp::*__p_)() const;
 public:
@@ -1223,7 +1238,7 @@ public:
 };
 
 template <class _Sp, class _Tp, class _Ap>
-class _LIBCPP_TEMPLATE_VIS const_mem_fun1_ref_t
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_DEPRECATED_IN_CXX11 const_mem_fun1_ref_t
     : public binary_function<_Tp, _Ap, _Sp>
 {
     _Sp (_Tp::*__p_)(_Ap) const;
@@ -1235,13 +1250,13 @@ public:
 };
 
 template <class _Sp, class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_DEPRECATED_IN_CXX11 inline _LIBCPP_INLINE_VISIBILITY
 const_mem_fun_ref_t<_Sp,_Tp>
 mem_fun_ref(_Sp (_Tp::*__f)() const)
     {return const_mem_fun_ref_t<_Sp,_Tp>(__f);}
 
 template <class _Sp, class _Tp, class _Ap>
-inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_DEPRECATED_IN_CXX11 inline _LIBCPP_INLINE_VISIBILITY
 const_mem_fun1_ref_t<_Sp,_Tp,_Ap>
 mem_fun_ref(_Sp (_Tp::*__f)(_Ap) const)
     {return const_mem_fun1_ref_t<_Sp,_Tp,_Ap>(__f);}
@@ -1405,7 +1420,7 @@ void __throw_bad_function_call()
 #ifndef _LIBCPP_NO_EXCEPTIONS
     throw bad_function_call();
 #else
-	_VSTD::abort();
+    _VSTD::abort();
 #endif
 }
 
@@ -1458,6 +1473,81 @@ bool __not_null(function<_Fp> const& __f) { return !!__f; }
 
 namespace __function {
 
+// __alloc_func holds a functor and an allocator.
+
+template <class _Fp, class _Ap, class _FB> class __alloc_func;
+
+template <class _Fp, class _Ap, class _Rp, class... _ArgTypes>
+class __alloc_func<_Fp, _Ap, _Rp(_ArgTypes...)>
+{
+    __compressed_pair<_Fp, _Ap> __f_;
+
+  public:
+    typedef _Fp _Target;
+    typedef _Ap _Alloc;
+
+    _LIBCPP_INLINE_VISIBILITY
+    const _Target& __target() const { return __f_.first(); }
+
+    _LIBCPP_INLINE_VISIBILITY
+    const _Alloc& __allocator() const { return __f_.second(); }
+
+    _LIBCPP_INLINE_VISIBILITY
+    explicit __alloc_func(_Target&& __f)
+        : __f_(piecewise_construct, _VSTD::forward_as_tuple(_VSTD::move(__f)),
+               _VSTD::forward_as_tuple())
+    {
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    explicit __alloc_func(const _Target& __f, const _Alloc& __a)
+        : __f_(piecewise_construct, _VSTD::forward_as_tuple(__f),
+               _VSTD::forward_as_tuple(__a))
+    {
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    explicit __alloc_func(const _Target& __f, _Alloc&& __a)
+        : __f_(piecewise_construct, _VSTD::forward_as_tuple(__f),
+               _VSTD::forward_as_tuple(_VSTD::move(__a)))
+    {
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    explicit __alloc_func(_Target&& __f, _Alloc&& __a)
+        : __f_(piecewise_construct, _VSTD::forward_as_tuple(_VSTD::move(__f)),
+               _VSTD::forward_as_tuple(_VSTD::move(__a)))
+    {
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    _Rp operator()(_ArgTypes&&... __arg)
+    {
+        typedef __invoke_void_return_wrapper<_Rp> _Invoker;
+        return _Invoker::__call(__f_.first(),
+                                _VSTD::forward<_ArgTypes>(__arg)...);
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    __alloc_func* __clone() const
+    {
+        typedef allocator_traits<_Alloc> __alloc_traits;
+        typedef
+            typename __rebind_alloc_helper<__alloc_traits, __alloc_func>::type
+                _AA;
+        _AA __a(__f_.second());
+        typedef __allocator_destructor<_AA> _Dp;
+        unique_ptr<__alloc_func, _Dp> __hold(__a.allocate(1), _Dp(__a, 1));
+        ::new ((void*)__hold.get()) __alloc_func(__f_.first(), _Alloc(__a));
+        return __hold.release();
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    void destroy() _NOEXCEPT { __f_.~__compressed_pair<_Target, _Alloc>(); }
+};
+
+// __base provides an abstract interface for copyable functors.
+
 template<class _Fp> class __base;
 
 template<class _Rp, class ..._ArgTypes>
@@ -1479,37 +1569,37 @@ public:
 #endif  // _LIBCPP_NO_RTTI
 };
 
+// __func implements __base for a given functor type.
+
 template<class _FD, class _Alloc, class _FB> class __func;
 
 template<class _Fp, class _Alloc, class _Rp, class ..._ArgTypes>
 class __func<_Fp, _Alloc, _Rp(_ArgTypes...)>
     : public  __base<_Rp(_ArgTypes...)>
 {
-    __compressed_pair<_Fp, _Alloc> __f_;
+    __alloc_func<_Fp, _Alloc, _Rp(_ArgTypes...)> __f_;
 public:
     _LIBCPP_INLINE_VISIBILITY
     explicit __func(_Fp&& __f)
-        : __f_(piecewise_construct, _VSTD::forward_as_tuple(_VSTD::move(__f)),
-                                    _VSTD::forward_as_tuple()) {}
+        : __f_(_VSTD::move(__f)) {}
+
     _LIBCPP_INLINE_VISIBILITY
     explicit __func(const _Fp& __f, const _Alloc& __a)
-        : __f_(piecewise_construct, _VSTD::forward_as_tuple(__f),
-                                    _VSTD::forward_as_tuple(__a)) {}
+        : __f_(__f, __a) {}
 
     _LIBCPP_INLINE_VISIBILITY
     explicit __func(const _Fp& __f, _Alloc&& __a)
-        : __f_(piecewise_construct, _VSTD::forward_as_tuple(__f),
-                                    _VSTD::forward_as_tuple(_VSTD::move(__a))) {}
+        : __f_(__f, _VSTD::move(__a)) {}
 
     _LIBCPP_INLINE_VISIBILITY
     explicit __func(_Fp&& __f, _Alloc&& __a)
-        : __f_(piecewise_construct, _VSTD::forward_as_tuple(_VSTD::move(__f)),
-                                    _VSTD::forward_as_tuple(_VSTD::move(__a))) {}
+        : __f_(_VSTD::move(__f), _VSTD::move(__a)) {}
+
     virtual __base<_Rp(_ArgTypes...)>* __clone() const;
     virtual void __clone(__base<_Rp(_ArgTypes...)>*) const;
     virtual void destroy() _NOEXCEPT;
     virtual void destroy_deallocate() _NOEXCEPT;
-    virtual _Rp operator()(_ArgTypes&& ... __arg);
+    virtual _Rp operator()(_ArgTypes&&... __arg);
 #ifndef _LIBCPP_NO_RTTI
     virtual const void* target(const type_info&) const _NOEXCEPT;
     virtual const std::type_info& target_type() const _NOEXCEPT;
@@ -1522,10 +1612,10 @@ __func<_Fp, _Alloc, _Rp(_ArgTypes...)>::__clone() const
 {
     typedef allocator_traits<_Alloc> __alloc_traits;
     typedef typename __rebind_alloc_helper<__alloc_traits, __func>::type _Ap;
-    _Ap __a(__f_.second());
+    _Ap __a(__f_.__allocator());
     typedef __allocator_destructor<_Ap> _Dp;
     unique_ptr<__func, _Dp> __hold(__a.allocate(1), _Dp(__a, 1));
-    ::new (__hold.get()) __func(__f_.first(), _Alloc(__a));
+    ::new ((void*)__hold.get()) __func(__f_.__target(), _Alloc(__a));
     return __hold.release();
 }
 
@@ -1533,14 +1623,14 @@ template<class _Fp, class _Alloc, class _Rp, class ..._ArgTypes>
 void
 __func<_Fp, _Alloc, _Rp(_ArgTypes...)>::__clone(__base<_Rp(_ArgTypes...)>* __p) const
 {
-    ::new (__p) __func(__f_.first(), __f_.second());
+    ::new (__p) __func(__f_.__target(), __f_.__allocator());
 }
 
 template<class _Fp, class _Alloc, class _Rp, class ..._ArgTypes>
 void
 __func<_Fp, _Alloc, _Rp(_ArgTypes...)>::destroy() _NOEXCEPT
 {
-    __f_.~__compressed_pair<_Fp, _Alloc>();
+    __f_.destroy();
 }
 
 template<class _Fp, class _Alloc, class _Rp, class ..._ArgTypes>
@@ -1549,8 +1639,8 @@ __func<_Fp, _Alloc, _Rp(_ArgTypes...)>::destroy_deallocate() _NOEXCEPT
 {
     typedef allocator_traits<_Alloc> __alloc_traits;
     typedef typename __rebind_alloc_helper<__alloc_traits, __func>::type _Ap;
-    _Ap __a(__f_.second());
-    __f_.~__compressed_pair<_Fp, _Alloc>();
+    _Ap __a(__f_.__allocator());
+    __f_.destroy();
     __a.deallocate(this, 1);
 }
 
@@ -1558,8 +1648,7 @@ template<class _Fp, class _Alloc, class _Rp, class ..._ArgTypes>
 _Rp
 __func<_Fp, _Alloc, _Rp(_ArgTypes...)>::operator()(_ArgTypes&& ... __arg)
 {
-    typedef __invoke_void_return_wrapper<_Rp> _Invoker;
-    return _Invoker::__call(__f_.first(), _VSTD::forward<_ArgTypes>(__arg)...);
+    return __f_(_VSTD::forward<_ArgTypes>(__arg)...);
 }
 
 #ifndef _LIBCPP_NO_RTTI
@@ -1569,7 +1658,7 @@ const void*
 __func<_Fp, _Alloc, _Rp(_ArgTypes...)>::target(const type_info& __ti) const _NOEXCEPT
 {
     if (__ti == typeid(_Fp))
-        return &__f_.first();
+        return &__f_.__target();
     return (const void*)0;
 }
 
@@ -1582,6 +1671,493 @@ __func<_Fp, _Alloc, _Rp(_ArgTypes...)>::target_type() const _NOEXCEPT
 
 #endif  // _LIBCPP_NO_RTTI
 
+// __value_func creates a value-type from a __func.
+
+template <class _Fp> class __value_func;
+
+template <class _Rp, class... _ArgTypes> class __value_func<_Rp(_ArgTypes...)>
+{
+    typename aligned_storage<3 * sizeof(void*)>::type __buf_;
+
+    typedef __base<_Rp(_ArgTypes...)> __func;
+    __func* __f_;
+
+    _LIBCPP_NO_CFI static __func* __as_base(void* p)
+    {
+        return reinterpret_cast<__func*>(p);
+    }
+
+  public:
+    _LIBCPP_INLINE_VISIBILITY
+    __value_func() _NOEXCEPT : __f_(0) {}
+
+    template <class _Fp, class _Alloc>
+    _LIBCPP_INLINE_VISIBILITY __value_func(_Fp&& __f, const _Alloc __a)
+        : __f_(0)
+    {
+        typedef allocator_traits<_Alloc> __alloc_traits;
+        typedef __function::__func<_Fp, _Alloc, _Rp(_ArgTypes...)> _Fun;
+        typedef typename __rebind_alloc_helper<__alloc_traits, _Fun>::type
+            _FunAlloc;
+
+        if (__function::__not_null(__f))
+        {
+            _FunAlloc __af(__a);
+            if (sizeof(_Fun) <= sizeof(__buf_) &&
+                is_nothrow_copy_constructible<_Fp>::value &&
+                is_nothrow_copy_constructible<_FunAlloc>::value)
+            {
+                __f_ =
+                    ::new ((void*)&__buf_) _Fun(_VSTD::move(__f), _Alloc(__af));
+            }
+            else
+            {
+                typedef __allocator_destructor<_FunAlloc> _Dp;
+                unique_ptr<__func, _Dp> __hold(__af.allocate(1), _Dp(__af, 1));
+                ::new ((void*)__hold.get()) _Fun(_VSTD::move(__f), _Alloc(__a));
+                __f_ = __hold.release();
+            }
+        }
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    __value_func(const __value_func& __f)
+    {
+        if (__f.__f_ == 0)
+            __f_ = 0;
+        else if ((void*)__f.__f_ == &__f.__buf_)
+        {
+            __f_ = __as_base(&__buf_);
+            __f.__f_->__clone(__f_);
+        }
+        else
+            __f_ = __f.__f_->__clone();
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    __value_func(__value_func&& __f) _NOEXCEPT
+    {
+        if (__f.__f_ == 0)
+            __f_ = 0;
+        else if ((void*)__f.__f_ == &__f.__buf_)
+        {
+            __f_ = __as_base(&__buf_);
+            __f.__f_->__clone(__f_);
+        }
+        else
+        {
+            __f_ = __f.__f_;
+            __f.__f_ = 0;
+        }
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    ~__value_func()
+    {
+        if ((void*)__f_ == &__buf_)
+            __f_->destroy();
+        else if (__f_)
+            __f_->destroy_deallocate();
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    __value_func& operator=(__value_func&& __f)
+    {
+        *this = nullptr;
+        if (__f.__f_ == 0)
+            __f_ = 0;
+        else if ((void*)__f.__f_ == &__f.__buf_)
+        {
+            __f_ = __as_base(&__buf_);
+            __f.__f_->__clone(__f_);
+        }
+        else
+        {
+            __f_ = __f.__f_;
+            __f.__f_ = 0;
+        }
+        return *this;
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    __value_func& operator=(nullptr_t)
+    {
+        __func* __f = __f_;
+        __f_ = 0;
+        if ((void*)__f == &__buf_)
+            __f->destroy();
+        else if (__f)
+            __f->destroy_deallocate();
+        return *this;
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    _Rp operator()(_ArgTypes&&... __args) const
+    {
+        if (__f_ == 0)
+            __throw_bad_function_call();
+        return (*__f_)(_VSTD::forward<_ArgTypes>(__args)...);
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    void swap(__value_func& __f) _NOEXCEPT
+    {
+        if (&__f == this)
+            return;
+        if ((void*)__f_ == &__buf_ && (void*)__f.__f_ == &__f.__buf_)
+        {
+            typename aligned_storage<sizeof(__buf_)>::type __tempbuf;
+            __func* __t = __as_base(&__tempbuf);
+            __f_->__clone(__t);
+            __f_->destroy();
+            __f_ = 0;
+            __f.__f_->__clone(__as_base(&__buf_));
+            __f.__f_->destroy();
+            __f.__f_ = 0;
+            __f_ = __as_base(&__buf_);
+            __t->__clone(__as_base(&__f.__buf_));
+            __t->destroy();
+            __f.__f_ = __as_base(&__f.__buf_);
+        }
+        else if ((void*)__f_ == &__buf_)
+        {
+            __f_->__clone(__as_base(&__f.__buf_));
+            __f_->destroy();
+            __f_ = __f.__f_;
+            __f.__f_ = __as_base(&__f.__buf_);
+        }
+        else if ((void*)__f.__f_ == &__f.__buf_)
+        {
+            __f.__f_->__clone(__as_base(&__buf_));
+            __f.__f_->destroy();
+            __f.__f_ = __f_;
+            __f_ = __as_base(&__buf_);
+        }
+        else
+            _VSTD::swap(__f_, __f.__f_);
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_EXPLICIT operator bool() const _NOEXCEPT { return __f_ != 0; }
+
+#ifndef _LIBCPP_NO_RTTI
+    _LIBCPP_INLINE_VISIBILITY
+    const std::type_info& target_type() const _NOEXCEPT
+    {
+        if (__f_ == 0)
+            return typeid(void);
+        return __f_->target_type();
+    }
+
+    template <typename _Tp>
+    _LIBCPP_INLINE_VISIBILITY const _Tp* target() const _NOEXCEPT
+    {
+        if (__f_ == 0)
+            return 0;
+        return (const _Tp*)__f_->target(typeid(_Tp));
+    }
+#endif // _LIBCPP_NO_RTTI
+};
+
+// Storage for a functor object, to be used with __policy to manage copy and
+// destruction.
+union __policy_storage
+{
+    mutable char __small[sizeof(void*) * 2];
+    void* __large;
+};
+
+// True if _Fun can safely be held in __policy_storage.__small.
+template <typename _Fun>
+struct __use_small_storage
+    : public _VSTD::integral_constant<
+          bool, sizeof(_Fun) <= sizeof(__policy_storage) &&
+                    _LIBCPP_ALIGNOF(_Fun) <= _LIBCPP_ALIGNOF(__policy_storage) &&
+                    _VSTD::is_trivially_copy_constructible<_Fun>::value &&
+                    _VSTD::is_trivially_destructible<_Fun>::value> {};
+
+// Policy contains information about how to copy, destroy, and move the
+// underlying functor. You can think of it as a vtable of sorts.
+struct __policy
+{
+    // Used to copy or destroy __large values. null for trivial objects.
+    void* (*const __clone)(const void*);
+    void (*const __destroy)(void*);
+
+    // True if this is the null policy (no value).
+    const bool __is_null;
+
+    // The target type. May be null if RTTI is disabled.
+    const std::type_info* const __type_info;
+
+    // Returns a pointer to a static policy object suitable for the functor
+    // type.
+    template <typename _Fun>
+    _LIBCPP_INLINE_VISIBILITY static const __policy* __create()
+    {
+        return __choose_policy<_Fun>(__use_small_storage<_Fun>());
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    static const __policy* __create_empty()
+    {
+        static const _LIBCPP_CONSTEXPR __policy __policy_ = {nullptr, nullptr,
+                                                             true,
+#ifndef _LIBCPP_NO_RTTI
+                                                             &typeid(void)
+#else
+                                                             nullptr
+#endif
+        };
+        return &__policy_;
+    }
+
+  private:
+    template <typename _Fun> static void* __large_clone(const void* __s)
+    {
+        const _Fun* __f = static_cast<const _Fun*>(__s);
+        return __f->__clone();
+    }
+
+    template <typename _Fun> static void __large_destroy(void* __s)
+    {
+        typedef allocator_traits<typename _Fun::_Alloc> __alloc_traits;
+        typedef typename __rebind_alloc_helper<__alloc_traits, _Fun>::type
+            _FunAlloc;
+        _Fun* __f = static_cast<_Fun*>(__s);
+        _FunAlloc __a(__f->__allocator());
+        __f->destroy();
+        __a.deallocate(__f, 1);
+    }
+
+    template <typename _Fun>
+    _LIBCPP_INLINE_VISIBILITY static const __policy*
+        __choose_policy(/* is_small = */ false_type)
+    {
+        static const _LIBCPP_CONSTEXPR __policy __policy_ = {
+            &__large_clone<_Fun>, &__large_destroy<_Fun>, false,
+#ifndef _LIBCPP_NO_RTTI
+            &typeid(typename _Fun::_Target)
+#else
+            nullptr
+#endif
+        };
+        return &__policy_;
+    }
+
+    template <typename _Fun>
+    _LIBCPP_INLINE_VISIBILITY static const __policy*
+        __choose_policy(/* is_small = */ true_type)
+    {
+        static const _LIBCPP_CONSTEXPR __policy __policy_ = {
+            nullptr, nullptr, false,
+#ifndef _LIBCPP_NO_RTTI
+            &typeid(typename _Fun::_Target)
+#else
+            nullptr
+#endif
+        };
+        return &__policy_;
+    }
+};
+
+// Used to choose between perfect forwarding or pass-by-value. Pass-by-value is
+// faster for types that can be passed in registers.
+template <typename _Tp>
+using __fast_forward =
+    typename _VSTD::conditional<_VSTD::is_scalar<_Tp>::value, _Tp, _Tp&&>::type;
+
+// __policy_invoker calls an instance of __alloc_func held in __policy_storage.
+
+template <class _Fp> struct __policy_invoker;
+
+template <class _Rp, class... _ArgTypes>
+struct __policy_invoker<_Rp(_ArgTypes...)>
+{
+    typedef _Rp (*__Call)(const __policy_storage*,
+                          __fast_forward<_ArgTypes>...);
+
+    __Call __call_;
+
+    // Creates an invoker that throws bad_function_call.
+    _LIBCPP_INLINE_VISIBILITY
+    __policy_invoker() : __call_(&__call_empty) {}
+
+    // Creates an invoker that calls the given instance of __func.
+    template <typename _Fun>
+    _LIBCPP_INLINE_VISIBILITY static __policy_invoker __create()
+    {
+        return __policy_invoker(&__call_impl<_Fun>);
+    }
+
+  private:
+    _LIBCPP_INLINE_VISIBILITY
+    explicit __policy_invoker(__Call __c) : __call_(__c) {}
+
+    static _Rp __call_empty(const __policy_storage*,
+                            __fast_forward<_ArgTypes>...)
+    {
+        __throw_bad_function_call();
+    }
+
+    template <typename _Fun>
+    static _Rp __call_impl(const __policy_storage* __buf,
+                           __fast_forward<_ArgTypes>... __args)
+    {
+        _Fun* __f = reinterpret_cast<_Fun*>(__use_small_storage<_Fun>::value
+                                                ? &__buf->__small
+                                                : __buf->__large);
+        return (*__f)(_VSTD::forward<_ArgTypes>(__args)...);
+    }
+};
+
+// __policy_func uses a __policy and __policy_invoker to create a type-erased,
+// copyable functor.
+
+template <class _Fp> class __policy_func;
+
+template <class _Rp, class... _ArgTypes> class __policy_func<_Rp(_ArgTypes...)>
+{
+    // Inline storage for small objects.
+    __policy_storage __buf_;
+
+    // Calls the value stored in __buf_. This could technically be part of
+    // policy, but storing it here eliminates a level of indirection inside
+    // operator().
+    typedef __function::__policy_invoker<_Rp(_ArgTypes...)> __invoker;
+    __invoker __invoker_;
+
+    // The policy that describes how to move / copy / destroy __buf_. Never
+    // null, even if the function is empty.
+    const __policy* __policy_;
+
+  public:
+    _LIBCPP_INLINE_VISIBILITY
+    __policy_func() : __policy_(__policy::__create_empty()) {}
+
+    template <class _Fp, class _Alloc>
+    _LIBCPP_INLINE_VISIBILITY __policy_func(_Fp&& __f, const _Alloc& __a)
+        : __policy_(__policy::__create_empty())
+    {
+        typedef __alloc_func<_Fp, _Alloc, _Rp(_ArgTypes...)> _Fun;
+        typedef allocator_traits<_Alloc> __alloc_traits;
+        typedef typename __rebind_alloc_helper<__alloc_traits, _Fun>::type
+            _FunAlloc;
+
+        if (__function::__not_null(__f))
+        {
+            __invoker_ = __invoker::template __create<_Fun>();
+            __policy_ = __policy::__create<_Fun>();
+
+            _FunAlloc __af(__a);
+            if (__use_small_storage<_Fun>())
+            {
+                ::new ((void*)&__buf_.__small)
+                    _Fun(_VSTD::move(__f), _Alloc(__af));
+            }
+            else
+            {
+                typedef __allocator_destructor<_FunAlloc> _Dp;
+                unique_ptr<_Fun, _Dp> __hold(__af.allocate(1), _Dp(__af, 1));
+                ::new ((void*)__hold.get())
+                    _Fun(_VSTD::move(__f), _Alloc(__af));
+                __buf_.__large = __hold.release();
+            }
+        }
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    __policy_func(const __policy_func& __f)
+        : __buf_(__f.__buf_), __invoker_(__f.__invoker_),
+          __policy_(__f.__policy_)
+    {
+        if (__policy_->__clone)
+            __buf_.__large = __policy_->__clone(__f.__buf_.__large);
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    __policy_func(__policy_func&& __f)
+        : __buf_(__f.__buf_), __invoker_(__f.__invoker_),
+          __policy_(__f.__policy_)
+    {
+        if (__policy_->__destroy)
+        {
+            __f.__policy_ = __policy::__create_empty();
+            __f.__invoker_ = __invoker();
+        }
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    ~__policy_func()
+    {
+        if (__policy_->__destroy)
+            __policy_->__destroy(__buf_.__large);
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    __policy_func& operator=(__policy_func&& __f)
+    {
+        *this = nullptr;
+        __buf_ = __f.__buf_;
+        __invoker_ = __f.__invoker_;
+        __policy_ = __f.__policy_;
+        __f.__policy_ = __policy::__create_empty();
+        __f.__invoker_ = __invoker();
+        return *this;
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    __policy_func& operator=(nullptr_t)
+    {
+        const __policy* __p = __policy_;
+        __policy_ = __policy::__create_empty();
+        __invoker_ = __invoker();
+        if (__p->__destroy)
+            __p->__destroy(__buf_.__large);
+        return *this;
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    _Rp operator()(_ArgTypes&&... __args) const
+    {
+        return __invoker_.__call_(_VSTD::addressof(__buf_),
+                                  _VSTD::forward<_ArgTypes>(__args)...);
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    void swap(__policy_func& __f)
+    {
+        _VSTD::swap(__invoker_, __f.__invoker_);
+        _VSTD::swap(__policy_, __f.__policy_);
+        _VSTD::swap(__buf_, __f.__buf_);
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    explicit operator bool() const _NOEXCEPT
+    {
+        return !__policy_->__is_null;
+    }
+
+#ifndef _LIBCPP_NO_RTTI
+    _LIBCPP_INLINE_VISIBILITY
+    const std::type_info& target_type() const _NOEXCEPT
+    {
+        return *__policy_->__type_info;
+    }
+
+    template <typename _Tp>
+    _LIBCPP_INLINE_VISIBILITY const _Tp* target() const _NOEXCEPT
+    {
+        if (__policy_->__is_null || typeid(_Tp) != *__policy_->__type_info)
+            return nullptr;
+        if (__policy_->__clone) // Out of line storage.
+            return reinterpret_cast<const _Tp*>(__buf_.__large);
+        else
+            return reinterpret_cast<const _Tp*>(&__buf_.__small);
+    }
+#endif // _LIBCPP_NO_RTTI
+};
+
 }  // __function
 
 template<class _Rp, class ..._ArgTypes>
@@ -1589,13 +2165,13 @@ class _LIBCPP_TEMPLATE_VIS function<_Rp(_ArgTypes...)>
     : public __function::__maybe_derive_from_unary_function<_Rp(_ArgTypes...)>,
       public __function::__maybe_derive_from_binary_function<_Rp(_ArgTypes...)>
 {
-    typedef __function::__base<_Rp(_ArgTypes...)> __base;
-    typename aligned_storage<3*sizeof(void*)>::type __buf_;
-    __base* __f_;
+#ifndef _LIBCPP_ABI_OPTIMIZED_FUNCTION
+    typedef __function::__value_func<_Rp(_ArgTypes...)> __func;
+#else
+    typedef __function::__policy_func<_Rp(_ArgTypes...)> __func;
+#endif
 
-    _LIBCPP_NO_CFI static __base *__as_base(void *p) {
-      return reinterpret_cast<__base*>(p);
-    }
+    __func __f_;
 
     template <class _Fp, bool = __lazy_and<
         integral_constant<bool, !is_same<__uncvref_t<_Fp>, function>::value>,
@@ -1622,9 +2198,9 @@ public:
 
     // construct/copy/destroy:
     _LIBCPP_INLINE_VISIBILITY
-    function() _NOEXCEPT : __f_(0) {}
+    function() _NOEXCEPT { }
     _LIBCPP_INLINE_VISIBILITY
-    function(nullptr_t) _NOEXCEPT : __f_(0) {}
+    function(nullptr_t) _NOEXCEPT {}
     function(const function&);
     function(function&&) _NOEXCEPT;
     template<class _Fp, class = _EnableIfCallable<_Fp>>
@@ -1633,10 +2209,10 @@ public:
 #if _LIBCPP_STD_VER <= 14
     template<class _Alloc>
       _LIBCPP_INLINE_VISIBILITY
-      function(allocator_arg_t, const _Alloc&) _NOEXCEPT : __f_(0) {}
+      function(allocator_arg_t, const _Alloc&) _NOEXCEPT {}
     template<class _Alloc>
       _LIBCPP_INLINE_VISIBILITY
-      function(allocator_arg_t, const _Alloc&, nullptr_t) _NOEXCEPT : __f_(0) {}
+      function(allocator_arg_t, const _Alloc&, nullptr_t) _NOEXCEPT {}
     template<class _Alloc>
       function(allocator_arg_t, const _Alloc&, const function&);
     template<class _Alloc>
@@ -1665,7 +2241,9 @@ public:
 
     // function capacity:
     _LIBCPP_INLINE_VISIBILITY
-        _LIBCPP_EXPLICIT operator bool() const _NOEXCEPT {return __f_;}
+    _LIBCPP_EXPLICIT operator bool() const _NOEXCEPT {
+      return static_cast<bool>(__f_);
+    }
 
     // deleted overloads close possible hole in the type system
     template<class _R2, class... _ArgTypes2>
@@ -1685,125 +2263,38 @@ public:
 };
 
 template<class _Rp, class ..._ArgTypes>
-function<_Rp(_ArgTypes...)>::function(const function& __f)
-{
-    if (__f.__f_ == 0)
-        __f_ = 0;
-    else if ((void *)__f.__f_ == &__f.__buf_)
-    {
-        __f_ = __as_base(&__buf_);
-        __f.__f_->__clone(__f_);
-    }
-    else
-        __f_ = __f.__f_->__clone();
-}
+function<_Rp(_ArgTypes...)>::function(const function& __f) : __f_(__f.__f_) {}
 
 #if _LIBCPP_STD_VER <= 14
 template<class _Rp, class ..._ArgTypes>
 template <class _Alloc>
 function<_Rp(_ArgTypes...)>::function(allocator_arg_t, const _Alloc&,
-                                     const function& __f)
-{
-    if (__f.__f_ == 0)
-        __f_ = 0;
-    else if ((void *)__f.__f_ == &__f.__buf_)
-    {
-        __f_ = __as_base(&__buf_);
-        __f.__f_->__clone(__f_);
-    }
-    else
-        __f_ = __f.__f_->__clone();
-}
+                                     const function& __f) : __f_(__f.__f_) {}
 #endif
 
-template<class _Rp, class ..._ArgTypes>
+template <class _Rp, class... _ArgTypes>
 function<_Rp(_ArgTypes...)>::function(function&& __f) _NOEXCEPT
-{
-    if (__f.__f_ == 0)
-        __f_ = 0;
-    else if ((void *)__f.__f_ == &__f.__buf_)
-    {
-        __f_ = __as_base(&__buf_);
-        __f.__f_->__clone(__f_);
-    }
-    else
-    {
-        __f_ = __f.__f_;
-        __f.__f_ = 0;
-    }
-}
+    : __f_(_VSTD::move(__f.__f_)) {}
 
 #if _LIBCPP_STD_VER <= 14
 template<class _Rp, class ..._ArgTypes>
 template <class _Alloc>
 function<_Rp(_ArgTypes...)>::function(allocator_arg_t, const _Alloc&,
-                                     function&& __f)
-{
-    if (__f.__f_ == 0)
-        __f_ = 0;
-    else if ((void *)__f.__f_ == &__f.__buf_)
-    {
-        __f_ = __as_base(&__buf_);
-        __f.__f_->__clone(__f_);
-    }
-    else
-    {
-        __f_ = __f.__f_;
-        __f.__f_ = 0;
-    }
-}
+                                      function&& __f)
+    : __f_(_VSTD::move(__f.__f_)) {}
 #endif
 
-template<class _Rp, class ..._ArgTypes>
+template <class _Rp, class... _ArgTypes>
 template <class _Fp, class>
 function<_Rp(_ArgTypes...)>::function(_Fp __f)
-    : __f_(0)
-{
-    if (__function::__not_null(__f))
-    {
-        typedef __function::__func<_Fp, allocator<_Fp>, _Rp(_ArgTypes...)> _FF;
-        if (sizeof(_FF) <= sizeof(__buf_) && is_nothrow_copy_constructible<_Fp>::value)
-        {
-            __f_ = ::new((void*)&__buf_) _FF(_VSTD::move(__f));
-        }
-        else
-        {
-            typedef allocator<_FF> _Ap;
-            _Ap __a;
-            typedef __allocator_destructor<_Ap> _Dp;
-            unique_ptr<__base, _Dp> __hold(__a.allocate(1), _Dp(__a, 1));
-            ::new (__hold.get()) _FF(_VSTD::move(__f), allocator<_Fp>(__a));
-            __f_ = __hold.release();
-        }
-    }
-}
+    : __f_(_VSTD::move(__f), allocator<_Fp>()) {}
 
 #if _LIBCPP_STD_VER <= 14
-template<class _Rp, class ..._ArgTypes>
+template <class _Rp, class... _ArgTypes>
 template <class _Fp, class _Alloc, class>
-function<_Rp(_ArgTypes...)>::function(allocator_arg_t, const _Alloc& __a0, _Fp __f)
-    : __f_(0)
-{
-    typedef allocator_traits<_Alloc> __alloc_traits;
-    if (__function::__not_null(__f))
-    {
-        typedef __function::__func<_Fp, _Alloc, _Rp(_ArgTypes...)> _FF;
-        typedef typename __rebind_alloc_helper<__alloc_traits, _FF>::type _Ap;
-        _Ap __a(__a0);
-        if (sizeof(_FF) <= sizeof(__buf_) && 
-            is_nothrow_copy_constructible<_Fp>::value && is_nothrow_copy_constructible<_Ap>::value)
-        {
-            __f_ = ::new((void*)&__buf_) _FF(_VSTD::move(__f), _Alloc(__a));
-        }
-        else
-        {
-            typedef __allocator_destructor<_Ap> _Dp;
-            unique_ptr<__base, _Dp> __hold(__a.allocate(1), _Dp(__a, 1));
-            ::new (__hold.get()) _FF(_VSTD::move(__f), _Alloc(__a));
-            __f_ = __hold.release();
-        }
-    }
-}
+function<_Rp(_ArgTypes...)>::function(allocator_arg_t, const _Alloc& __a,
+                                      _Fp __f)
+    : __f_(_VSTD::move(__f), __a) {}
 #endif
 
 template<class _Rp, class ..._ArgTypes>
@@ -1818,19 +2309,7 @@ template<class _Rp, class ..._ArgTypes>
 function<_Rp(_ArgTypes...)>&
 function<_Rp(_ArgTypes...)>::operator=(function&& __f) _NOEXCEPT
 {
-    *this = nullptr;
-    if (__f.__f_ == 0)
-        __f_ = 0;
-    else if ((void *)__f.__f_ == &__f.__buf_)
-    {
-        __f_ = __as_base(&__buf_);
-        __f.__f_->__clone(__f_);
-    }
-    else
-    {
-        __f_ = __f.__f_;
-        __f.__f_ = 0;
-    }
+    __f_ = std::move(__f.__f_);
     return *this;
 }
 
@@ -1838,12 +2317,7 @@ template<class _Rp, class ..._ArgTypes>
 function<_Rp(_ArgTypes...)>&
 function<_Rp(_ArgTypes...)>::operator=(nullptr_t) _NOEXCEPT
 {
-    __base* __t = __f_;
-    __f_ = 0;
-    if ((void *)__t == &__buf_)
-        __t->destroy();
-    else if (__t)
-        __t->destroy_deallocate();
+    __f_ = nullptr;
     return *this;
 }
 
@@ -1857,60 +2331,20 @@ function<_Rp(_ArgTypes...)>::operator=(_Fp&& __f)
 }
 
 template<class _Rp, class ..._ArgTypes>
-function<_Rp(_ArgTypes...)>::~function()
-{
-    if ((void *)__f_ == &__buf_)
-        __f_->destroy();
-    else if (__f_)
-        __f_->destroy_deallocate();
-}
+function<_Rp(_ArgTypes...)>::~function() {}
 
 template<class _Rp, class ..._ArgTypes>
 void
 function<_Rp(_ArgTypes...)>::swap(function& __f) _NOEXCEPT
 {
-    if (_VSTD::addressof(__f) == this)
-      return;
-    if ((void *)__f_ == &__buf_ && (void *)__f.__f_ == &__f.__buf_)
-    {
-        typename aligned_storage<sizeof(__buf_)>::type __tempbuf;
-        __base* __t = __as_base(&__tempbuf);
-        __f_->__clone(__t);
-        __f_->destroy();
-        __f_ = 0;
-        __f.__f_->__clone(__as_base(&__buf_));
-        __f.__f_->destroy();
-        __f.__f_ = 0;
-        __f_ = __as_base(&__buf_);
-        __t->__clone(__as_base(&__f.__buf_));
-        __t->destroy();
-        __f.__f_ = __as_base(&__f.__buf_);
-    }
-    else if ((void *)__f_ == &__buf_)
-    {
-        __f_->__clone(__as_base(&__f.__buf_));
-        __f_->destroy();
-        __f_ = __f.__f_;
-        __f.__f_ = __as_base(&__f.__buf_);
-    }
-    else if ((void *)__f.__f_ == &__f.__buf_)
-    {
-        __f.__f_->__clone(__as_base(&__buf_));
-        __f.__f_->destroy();
-        __f.__f_ = __f_;
-        __f_ = __as_base(&__buf_);
-    }
-    else
-        _VSTD::swap(__f_, __f.__f_);
+    __f_.swap(__f.__f_);
 }
 
 template<class _Rp, class ..._ArgTypes>
 _Rp
 function<_Rp(_ArgTypes...)>::operator()(_ArgTypes... __arg) const
 {
-    if (__f_ == 0)
-        __throw_bad_function_call();
-    return (*__f_)(_VSTD::forward<_ArgTypes>(__arg)...);
+    return __f_(_VSTD::forward<_ArgTypes>(__arg)...);
 }
 
 #ifndef _LIBCPP_NO_RTTI
@@ -1919,9 +2353,7 @@ template<class _Rp, class ..._ArgTypes>
 const std::type_info&
 function<_Rp(_ArgTypes...)>::target_type() const _NOEXCEPT
 {
-    if (__f_ == 0)
-        return typeid(void);
-    return __f_->target_type();
+    return __f_.target_type();
 }
 
 template<class _Rp, class ..._ArgTypes>
@@ -1929,9 +2361,7 @@ template <typename _Tp>
 _Tp*
 function<_Rp(_ArgTypes...)>::target() _NOEXCEPT
 {
-    if (__f_ == 0)
-        return nullptr;
-    return (_Tp*) const_cast<void *>(__f_->target(typeid(_Tp)));
+    return (_Tp*)(__f_.template target<_Tp>());
 }
 
 template<class _Rp, class ..._ArgTypes>
@@ -1939,9 +2369,7 @@ template <typename _Tp>
 const _Tp*
 function<_Rp(_ArgTypes...)>::target() const _NOEXCEPT
 {
-    if (__f_ == 0)
-        return nullptr;
-    return (const _Tp*)__f_->target(typeid(_Tp));
+    return __f_.template target<_Tp>();
 }
 
 #endif  // _LIBCPP_NO_RTTI
@@ -2105,53 +2533,53 @@ __mu(_Ti& __ti, _Uj&)
 
 template <class _Ti, bool IsReferenceWrapper, bool IsBindEx, bool IsPh,
           class _TupleUj>
-struct ____mu_return;
+struct __mu_return_impl;
 
 template <bool _Invokable, class _Ti, class ..._Uj>
-struct ____mu_return_invokable  // false
+struct __mu_return_invokable  // false
 {
     typedef __nat type;
 };
 
 template <class _Ti, class ..._Uj>
-struct ____mu_return_invokable<true, _Ti, _Uj...>
+struct __mu_return_invokable<true, _Ti, _Uj...>
 {
     typedef typename __invoke_of<_Ti&, _Uj...>::type type;
 };
 
 template <class _Ti, class ..._Uj>
-struct ____mu_return<_Ti, false, true, false, tuple<_Uj...> >
-    : public ____mu_return_invokable<__invokable<_Ti&, _Uj...>::value, _Ti, _Uj...>
+struct __mu_return_impl<_Ti, false, true, false, tuple<_Uj...> >
+    : public __mu_return_invokable<__invokable<_Ti&, _Uj...>::value, _Ti, _Uj...>
 {
 };
 
 template <class _Ti, class _TupleUj>
-struct ____mu_return<_Ti, false, false, true, _TupleUj>
+struct __mu_return_impl<_Ti, false, false, true, _TupleUj>
 {
     typedef typename tuple_element<is_placeholder<_Ti>::value - 1,
                                    _TupleUj>::type&& type;
 };
 
 template <class _Ti, class _TupleUj>
-struct ____mu_return<_Ti, true, false, false, _TupleUj>
+struct __mu_return_impl<_Ti, true, false, false, _TupleUj>
 {
     typedef typename _Ti::type& type;
 };
 
 template <class _Ti, class _TupleUj>
-struct ____mu_return<_Ti, false, false, false, _TupleUj>
+struct __mu_return_impl<_Ti, false, false, false, _TupleUj>
 {
     typedef _Ti& type;
 };
 
 template <class _Ti, class _TupleUj>
 struct __mu_return
-    : public ____mu_return<_Ti,
-                           __is_reference_wrapper<_Ti>::value,
-                           is_bind_expression<_Ti>::value,
-                           0 < is_placeholder<_Ti>::value &&
-                           is_placeholder<_Ti>::value <= tuple_size<_TupleUj>::value,
-                           _TupleUj>
+    : public __mu_return_impl<_Ti,
+                              __is_reference_wrapper<_Ti>::value,
+                              is_bind_expression<_Ti>::value,
+                              0 < is_placeholder<_Ti>::value &&
+                              is_placeholder<_Ti>::value <= tuple_size<_TupleUj>::value,
+                              _TupleUj>
 {
 };
 
@@ -2340,8 +2768,6 @@ bind(_Fp&& __f, _BoundArgs&&... __bound_args)
 
 #if _LIBCPP_STD_VER > 14
 
-#define __cpp_lib_invoke 201411
-
 template <class _Fn, class ..._Args>
 result_of_t<_Fn&&(_Args&&...)>
 invoke(_Fn&& __f, _Args&&... __args)
@@ -2497,7 +2923,7 @@ template<class _ForwardIterator, class _BinaryPredicate = equal_to<>>
 class _LIBCPP_TYPE_VIS default_searcher {
 public:
     _LIBCPP_INLINE_VISIBILITY
-    default_searcher(_ForwardIterator __f, _ForwardIterator __l, 
+    default_searcher(_ForwardIterator __f, _ForwardIterator __l,
                        _BinaryPredicate __p = _BinaryPredicate())
         : __first_(__f), __last_(__l), __pred_(__p) {}
 
@@ -2519,6 +2945,26 @@ private:
 
 #endif // _LIBCPP_STD_VER > 14
 
+#if _LIBCPP_STD_VER > 17
+template <class _Tp>
+using unwrap_reference_t = typename unwrap_reference<_Tp>::type;
+
+template <class _Tp>
+using unwrap_ref_decay_t = typename unwrap_ref_decay<_Tp>::type;
+#endif // > C++17
+
+template <class _Container, class _Predicate>
+inline void __libcpp_erase_if_container( _Container& __c, _Predicate __pred)
+{
+	for (typename _Container::iterator __iter = __c.begin(), __last = __c.end(); __iter != __last;)
+	{
+		if (__pred(*__iter))
+			__iter = __c.erase(__iter);
+		else
+			++__iter;
+	}
+}
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif  // _LIBCPP_FUNCTIONAL
diff --git a/include/future b/include/future
index 0f6d42678ef5..b3ffc7e35177 100644
--- a/include/future
+++ b/include/future
@@ -556,13 +556,14 @@ public:
         {return (__state_ & __constructed) || (__exception_ != nullptr);}
 
     _LIBCPP_INLINE_VISIBILITY
-    void __set_future_attached()
-    {
+    void __attach_future() {
         lock_guard<mutex> __lk(__mut_);
+        bool __has_future_attached = (__state_ & __future_attached) != 0;
+        if (__has_future_attached)
+            __throw_future_error(future_errc::future_already_retrieved);
+        this->__add_shared();
         __state_ |= __future_attached;
     }
-    _LIBCPP_INLINE_VISIBILITY
-    bool __has_future_attached() const {return (__state_ & __future_attached) != 0;}
 
     _LIBCPP_INLINE_VISIBILITY
     void __set_deferred() {__state_ |= deferred;}
@@ -1154,10 +1155,7 @@ template <class _Rp>
 future<_Rp>::future(__assoc_state<_Rp>* __state)
     : __state_(__state)
 {
-    if (__state_->__has_future_attached())
-        __throw_future_error(future_errc::future_already_retrieved);
-    __state_->__add_shared();
-    __state_->__set_future_attached();
+    __state_->__attach_future();
 }
 
 struct __release_shared_count
@@ -1257,10 +1255,7 @@ template <class _Rp>
 future<_Rp&>::future(__assoc_state<_Rp&>* __state)
     : __state_(__state)
 {
-    if (__state_->__has_future_attached())
-        __throw_future_error(future_errc::future_already_retrieved);
-    __state_->__add_shared();
-    __state_->__set_future_attached();
+    __state_->__attach_future();
 }
 
 template <class _Rp>
diff --git a/include/iomanip b/include/iomanip
index a6bee736f45b..36c11167a44e 100644
--- a/include/iomanip
+++ b/include/iomanip
@@ -46,6 +46,7 @@ template <class charT, class traits, class Allocator>
 #include <__config>
 #include <__string>
 #include <istream>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
diff --git a/include/iosfwd b/include/iosfwd
index d4384859e418..31f1902e5f59 100644
--- a/include/iosfwd
+++ b/include/iosfwd
@@ -18,6 +18,12 @@ namespace std
 {
 
 template<class charT> struct char_traits;
+template<>            struct char_traits<char>;
+template<>            struct char_traits<char8_t>;  // C++20
+template<>            struct char_traits<char16_t>;
+template<>            struct char_traits<char32_t>;
+template<>            struct char_traits<wchar_t>;
+
 template<class T>     class allocator;
 
 class ios_base;
@@ -98,6 +104,14 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 class _LIBCPP_TYPE_VIS ios_base;
 
 template<class _CharT>  struct _LIBCPP_TEMPLATE_VIS char_traits;
+template<> struct char_traits<char>;
+#ifndef _LIBCPP_NO_HAS_CHAR8_T
+template<> struct char_traits<char8_t>;
+#endif
+template<> struct char_traits<char16_t>;
+template<> struct char_traits<char32_t>;
+template<> struct char_traits<wchar_t>;
+
 template<class _Tp>     class _LIBCPP_TEMPLATE_VIS allocator;
 
 template <class _CharT, class _Traits = char_traits<_CharT> >
@@ -175,6 +189,9 @@ typedef basic_fstream<wchar_t>       wfstream;
 template <class _State>             class _LIBCPP_TEMPLATE_VIS fpos;
 typedef fpos<mbstate_t>    streampos;
 typedef fpos<mbstate_t>    wstreampos;
+#ifndef _LIBCPP_NO_HAS_CHAR8_T
+typedef fpos<mbstate_t>    u8streampos;
+#endif
 #ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
 typedef fpos<mbstate_t>    u16streampos;
 typedef fpos<mbstate_t>    u32streampos;
diff --git a/include/istream b/include/istream
index 71c162b0d41a..30ee4f4b8710 100644
--- a/include/istream
+++ b/include/istream
@@ -160,6 +160,7 @@ template <class charT, class traits, class T>
 */
 
 #include <__config>
+#include <version>
 #include <ostream>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -186,7 +187,7 @@ public:
     typedef typename traits_type::off_type off_type;
 
     // 27.7.1.1.1 Constructor/destructor:
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     explicit basic_istream(basic_streambuf<char_type, traits_type>* __sb) : __gc_(0)
     { this->init(__sb); }
     virtual ~basic_istream();
@@ -200,7 +201,7 @@ protected:
     basic_istream& operator=(basic_istream&& __rhs);
 #endif
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     void swap(basic_istream& __rhs) {
       _VSTD::swap(__gc_, __rhs.__gc_);
       basic_ios<char_type, traits_type>::swap(__rhs);
@@ -216,16 +217,16 @@ public:
     class _LIBCPP_TEMPLATE_VIS sentry;
 
     // 27.7.1.2 Formatted input:
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     basic_istream& operator>>(basic_istream& (*__pf)(basic_istream&))
     { return __pf(*this); }
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     basic_istream& operator>>(basic_ios<char_type, traits_type>&
                               (*__pf)(basic_ios<char_type, traits_type>&))
     { __pf(*this); return *this; }
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     basic_istream& operator>>(ios_base& (*__pf)(ios_base&))
     { __pf(*this); return *this; }
 
@@ -249,7 +250,7 @@ public:
     streamsize gcount() const {return __gc_;}
     int_type get();
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     basic_istream& get(char_type& __c) {
       int_type __ch = get();
       if (__ch != traits_type::eof())
@@ -257,19 +258,19 @@ public:
       return *this;
     }
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     basic_istream& get(char_type* __s, streamsize __n)
     { return get(__s, __n, this->widen('\n')); }
 
     basic_istream& get(char_type* __s, streamsize __n, char_type __dlm);
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     basic_istream& get(basic_streambuf<char_type, traits_type>& __sb)
     { return get(__sb, this->widen('\n')); }
 
     basic_istream& get(basic_streambuf<char_type, traits_type>& __sb, char_type __dlm);
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     basic_istream& getline(char_type* __s, streamsize __n)
     { return getline(__s, __n, this->widen('\n')); }
 
@@ -517,8 +518,9 @@ basic_istream<_CharT, _Traits>::operator>>(int& __n)
 }
 
 template<class _CharT, class _Traits>
+_LIBCPP_INLINE_VISIBILITY
 basic_istream<_CharT, _Traits>&
-operator>>(basic_istream<_CharT, _Traits>& __is, _CharT* __s)
+__input_c_string(basic_istream<_CharT, _Traits>& __is, _CharT* __p, size_t __n)
 {
 #ifndef _LIBCPP_NO_EXCEPTIONS
     try
@@ -527,13 +529,10 @@ operator>>(basic_istream<_CharT, _Traits>& __is, _CharT* __s)
         typename basic_istream<_CharT, _Traits>::sentry __sen(__is);
         if (__sen)
         {
-            streamsize __n = __is.width();
-            if (__n <= 0)
-                __n = numeric_limits<streamsize>::max() / sizeof(_CharT) - 1;
-            streamsize __c = 0;
+            auto __s = __p;
             const ctype<_CharT>& __ct = use_facet<ctype<_CharT> >(__is.getloc());
             ios_base::iostate __err = ios_base::goodbit;
-            while (__c < __n-1)
+            while (__s != __p + (__n-1))
             {
                 typename _Traits::int_type __i = __is.rdbuf()->sgetc();
                 if (_Traits::eq_int_type(__i, _Traits::eof()))
@@ -545,12 +544,11 @@ operator>>(basic_istream<_CharT, _Traits>& __is, _CharT* __s)
                 if (__ct.is(__ct.space, __ch))
                     break;
                 *__s++ = __ch;
-                ++__c;
                  __is.rdbuf()->sbumpc();
             }
             *__s = _CharT();
             __is.width(0);
-            if (__c == 0)
+            if (__s == __p)
                __err |= ios_base::failbit;
             __is.setstate(__err);
         }
@@ -564,6 +562,48 @@ operator>>(basic_istream<_CharT, _Traits>& __is, _CharT* __s)
     return __is;
 }
 
+#if _LIBCPP_STD_VER > 17
+
+template<class _CharT, class _Traits, size_t _Np>
+inline _LIBCPP_INLINE_VISIBILITY
+basic_istream<_CharT, _Traits>&
+operator>>(basic_istream<_CharT, _Traits>& __is, _CharT (&__buf)[_Np])
+{
+    auto __n = _Np;
+    if (__is.width() > 0)
+        __n = _VSTD::min(size_t(__is.width()), _Np);
+    return _VSTD::__input_c_string(__is, __buf, __n);
+}
+
+template<class _Traits, size_t _Np>
+inline _LIBCPP_INLINE_VISIBILITY
+basic_istream<char, _Traits>&
+operator>>(basic_istream<char, _Traits>& __is, unsigned char (&__buf)[_Np])
+{
+    return __is >> (char(&)[_Np])__buf;
+}
+
+template<class _Traits, size_t _Np>
+inline _LIBCPP_INLINE_VISIBILITY
+basic_istream<char, _Traits>&
+operator>>(basic_istream<char, _Traits>& __is, signed char (&__buf)[_Np])
+{
+    return __is >> (char(&)[_Np])__buf;
+}
+
+#else
+
+template<class _CharT, class _Traits>
+inline _LIBCPP_INLINE_VISIBILITY
+basic_istream<_CharT, _Traits>&
+operator>>(basic_istream<_CharT, _Traits>& __is, _CharT* __s)
+{
+    streamsize __n = __is.width();
+    if (__n <= 0)
+        __n = numeric_limits<streamsize>::max() / sizeof(_CharT) - 1;
+    return _VSTD::__input_c_string(__is, __s, size_t(__n));
+}
+
 template<class _Traits>
 inline _LIBCPP_INLINE_VISIBILITY
 basic_istream<char, _Traits>&
@@ -580,6 +620,8 @@ operator>>(basic_istream<char, _Traits>& __is, signed char* __s)
     return __is >> (char*)__s;
 }
 
+#endif  // _LIBCPP_STD_VER > 17
+
 template<class _CharT, class _Traits>
 basic_istream<_CharT, _Traits>&
 operator>>(basic_istream<_CharT, _Traits>& __is, _CharT& __c)
@@ -1238,7 +1280,7 @@ public:
     typedef typename traits_type::off_type off_type;
 
     // constructor/destructor
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     explicit basic_iostream(basic_streambuf<char_type, traits_type>* __sb)
       : basic_istream<_CharT, _Traits>(__sb)
     {}
@@ -1253,7 +1295,7 @@ protected:
     inline _LIBCPP_INLINE_VISIBILITY
     basic_iostream& operator=(basic_iostream&& __rhs);
 #endif
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     void swap(basic_iostream& __rhs)
     { basic_istream<char_type, traits_type>::swap(__rhs); }
 public:
@@ -1463,7 +1505,7 @@ operator>>(basic_istream<_CharT, _Traits>& __is, bitset<_Size>& __x)
     return __is;
 }
 
-#ifndef _LIBCPP_AVAILABILITY_NO_STREAMS_EXTERN_TEMPLATE
+#ifndef _LIBCPP_DO_NOT_ASSUME_STREAMS_EXPLICIT_INSTANTIATION_IN_DYLIB
 _LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS basic_istream<char>)
 _LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS basic_istream<wchar_t>)
 _LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS basic_iostream<char>)
diff --git a/include/iterator b/include/iterator
index 9415e0b83967..bda177e11e6c 100644
--- a/include/iterator
+++ b/include/iterator
@@ -418,6 +418,7 @@ template <class E> constexpr const E* data(initializer_list<E> il) noexcept;
 #include <type_traits>
 #include <cstddef>
 #include <initializer_list>
+#include <version>
 #ifdef __APPLE__
 #include <Availability.h>
 #endif
@@ -437,6 +438,23 @@ struct _LIBCPP_TEMPLATE_VIS bidirectional_iterator_tag : public forward_iterator
 struct _LIBCPP_TEMPLATE_VIS random_access_iterator_tag : public bidirectional_iterator_tag {};
 
 template <class _Tp>
+struct __has_iterator_typedefs
+{
+private:
+    struct __two {char __lx; char __lxx;};
+    template <class _Up> static __two __test(...);
+    template <class _Up> static char __test(typename std::__void_t<typename _Up::iterator_category>::type* = 0,
+    										typename std::__void_t<typename _Up::difference_type>::type* = 0,
+    										typename std::__void_t<typename _Up::value_type>::type* = 0,
+    										typename std::__void_t<typename _Up::reference>::type* = 0,
+    										typename std::__void_t<typename _Up::pointer>::type* = 0
+    										);
+public:
+    static const bool value = sizeof(__test<_Tp>(0,0,0,0,0)) == 1;
+};
+
+
+template <class _Tp>
 struct __has_iterator_category
 {
 private:
@@ -478,7 +496,7 @@ struct __iterator_traits<_Iter, true>
 
 template <class _Iter>
 struct _LIBCPP_TEMPLATE_VIS iterator_traits
-    : __iterator_traits<_Iter, __has_iterator_category<_Iter>::value> {};
+    : __iterator_traits<_Iter, __has_iterator_typedefs<_Iter>::value> {};
 
 template<class _Tp>
 struct _LIBCPP_TEMPLATE_VIS iterator_traits<_Tp*>
diff --git a/include/limits b/include/limits
index f530507f7246..5ea9a9e6fdfa 100644
--- a/include/limits
+++ b/include/limits
@@ -82,6 +82,7 @@ template<> class numeric_limits<cv char>;
 template<> class numeric_limits<cv signed char>;
 template<> class numeric_limits<cv unsigned char>;
 template<> class numeric_limits<cv wchar_t>;
+template<> class numeric_limits<cv char8_t>; // C++20
 template<> class numeric_limits<cv char16_t>;
 template<> class numeric_limits<cv char32_t>;
 
@@ -118,6 +119,7 @@ template<> class numeric_limits<cv long double>;
 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
+#include <version>
 
 
 _LIBCPP_BEGIN_NAMESPACE_STD
diff --git a/include/list b/include/list
index d2e78cd66afb..c69e31d93a44 100644
--- a/include/list
+++ b/include/list
@@ -169,6 +169,11 @@ template <class T, class Alloc>
     void swap(list<T,Alloc>& x, list<T,Alloc>& y)
          noexcept(noexcept(x.swap(y)));
 
+template <class T, class Allocator, class U>
+    void erase(list<T, Allocator>& c, const U& value);       // C++20
+template <class T, class Allocator, class Predicate>
+    void erase_if(list<T, Allocator>& c, Predicate pred);    // C++20
+
 }  // std
 
 */
@@ -181,6 +186,7 @@ template <class T, class Alloc>
 #include <iterator>
 #include <algorithm>
 #include <type_traits>
+#include <version>
 
 #include <__debug>
 
@@ -1169,7 +1175,7 @@ list<_Tp, _Alloc>::__link_nodes_at_front(__link_pointer __f, __link_pointer __l)
     base::__end_.__next_ = __f;
 }
 
-// Link in nodes [__f, __l] at the front of the list
+// Link in nodes [__f, __l] at the back of the list
 template <class _Tp, class _Alloc>
 inline
 void
@@ -2208,7 +2214,7 @@ template <class _Comp>
 void
 list<_Tp, _Alloc>::merge(list& __c, _Comp __comp)
 {
-    if (this != &__c)
+    if (this != _VSTD::addressof(__c))
     {
         iterator __f1 = begin();
         iterator __e1 = end();
@@ -2449,6 +2455,18 @@ swap(list<_Tp, _Alloc>& __x, list<_Tp, _Alloc>& __y)
     __x.swap(__y);
 }
 
+#if _LIBCPP_STD_VER > 17
+template <class _Tp, class _Allocator, class _Predicate>
+inline _LIBCPP_INLINE_VISIBILITY
+void erase_if(list<_Tp, _Allocator>& __c, _Predicate __pred)
+{ __c.remove_if(__pred); }
+
+template <class _Tp, class _Allocator, class _Up>
+inline _LIBCPP_INLINE_VISIBILITY
+void erase(list<_Tp, _Allocator>& __c, const _Up& __v)
+{ _VSTD::erase_if(__c, [&](auto& __elem) { return __elem == __v; }); }
+#endif
+
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
diff --git a/include/locale b/include/locale
index e240799f3831..2043892fa2dc 100644
--- a/include/locale
+++ b/include/locale
@@ -187,6 +187,7 @@ template <class charT> class messages_byname;
 #include <streambuf>
 #include <iterator>
 #include <limits>
+#include <version>
 #ifndef __APPLE__
 #include <cstdarg>
 #endif
@@ -727,7 +728,7 @@ locale::id
 num_get<_CharT, _InputIterator>::id;
 
 template <class _Tp>
-_Tp
+_LIBCPP_HIDDEN _Tp
 __num_get_signed_integral(const char* __a, const char* __a_end,
                           ios_base::iostate& __err, int __base)
 {
@@ -762,7 +763,7 @@ __num_get_signed_integral(const char* __a, const char* __a_end,
 }
 
 template <class _Tp>
-_Tp
+_LIBCPP_HIDDEN _Tp
 __num_get_unsigned_integral(const char* __a, const char* __a_end,
                             ios_base::iostate& __err, int __base)
 {
@@ -2408,6 +2409,23 @@ private:
     string_type __analyze(char __fmt, const ctype<_CharT>&);
 };
 
+#define _LIBCPP_TIME_GET_STORAGE_EXPLICIT_INSTANTIATION(_CharT) \
+template <> _LIBCPP_FUNC_VIS time_base::dateorder __time_get_storage<_CharT>::__do_date_order() const; \
+template <> _LIBCPP_FUNC_VIS __time_get_storage<_CharT>::__time_get_storage(const char*); \
+template <> _LIBCPP_FUNC_VIS __time_get_storage<_CharT>::__time_get_storage(const string&); \
+template <> _LIBCPP_FUNC_VIS void __time_get_storage<_CharT>::init(const ctype<_CharT>&); \
+template <> _LIBCPP_FUNC_VIS __time_get_storage<_CharT>::string_type __time_get_storage<_CharT>::__analyze(char, const ctype<_CharT>&); \
+extern template _LIBCPP_FUNC_VIS time_base::dateorder __time_get_storage<_CharT>::__do_date_order() const; \
+extern template _LIBCPP_FUNC_VIS __time_get_storage<_CharT>::__time_get_storage(const char*); \
+extern template _LIBCPP_FUNC_VIS __time_get_storage<_CharT>::__time_get_storage(const string&); \
+extern template _LIBCPP_FUNC_VIS void __time_get_storage<_CharT>::init(const ctype<_CharT>&); \
+extern template _LIBCPP_FUNC_VIS __time_get_storage<_CharT>::string_type __time_get_storage<_CharT>::__analyze(char, const ctype<_CharT>&); \
+/**/
+
+_LIBCPP_TIME_GET_STORAGE_EXPLICIT_INSTANTIATION(char)
+_LIBCPP_TIME_GET_STORAGE_EXPLICIT_INSTANTIATION(wchar_t)
+#undef _LIBCPP_TIME_GET_STORAGE_EXPLICIT_INSTANTIATION
+
 template <class _CharT, class _InputIterator = istreambuf_iterator<_CharT> >
 class _LIBCPP_TEMPLATE_VIS time_get_byname
     : public time_get<_CharT, _InputIterator>,
@@ -3550,6 +3568,7 @@ messages<_CharT>::do_open(const basic_string<char>& __nm, const locale&) const
         __cat = static_cast<catalog>((static_cast<size_t>(__cat) >> 1));
     return __cat;
 #else // !_LIBCPP_HAS_CATOPEN
+    _LIBCPP_UNUSED_VAR(__nm);
     return -1;
 #endif // _LIBCPP_HAS_CATOPEN
 }
@@ -3573,6 +3592,9 @@ messages<_CharT>::do_get(catalog __c, int __set, int __msgid,
                                                         __n, __n + strlen(__n));
     return __w;
 #else // !_LIBCPP_HAS_CATOPEN
+    _LIBCPP_UNUSED_VAR(__c);
+    _LIBCPP_UNUSED_VAR(__set);
+    _LIBCPP_UNUSED_VAR(__msgid);
     return __dflt;
 #endif // _LIBCPP_HAS_CATOPEN
 }
@@ -3586,6 +3608,8 @@ messages<_CharT>::do_close(catalog __c) const
         __c <<= 1;
     nl_catd __cat = (nl_catd)__c;
     catclose(__cat);
+#else // !_LIBCPP_HAS_CATOPEN
+    _LIBCPP_UNUSED_VAR(__c);
 #endif // _LIBCPP_HAS_CATOPEN
 }
 
diff --git a/include/map b/include/map
index 559ec484aca0..616bb46cfccd 100644
--- a/include/map
+++ b/include/map
@@ -167,6 +167,15 @@ public:
     iterator  erase(const_iterator first, const_iterator last);
     void clear() noexcept;
 
+    template<class C2>
+      void merge(map<Key, T, C2, Allocator>& source);         // C++17
+    template<class C2>
+      void merge(map<Key, T, C2, Allocator>&& source);        // C++17
+    template<class C2>
+      void merge(multimap<Key, T, C2, Allocator>& source);    // C++17
+    template<class C2>
+      void merge(multimap<Key, T, C2, Allocator>&& source);   // C++17
+
     void swap(map& m)
         noexcept(allocator_traits<allocator_type>::is_always_equal::value &&
             is_nothrow_swappable<key_compare>::value); // C++17
@@ -245,6 +254,10 @@ void
 swap(map<Key, T, Compare, Allocator>& x, map<Key, T, Compare, Allocator>& y)
     noexcept(noexcept(x.swap(y)));
 
+template <class Key, class T, class Compare, class Allocator, class Predicate>
+  void erase_if(map<Key, T, Compare, Allocator>& c, Predicate pred);  // C++20
+
+
 template <class Key, class T, class Compare = less<Key>,
           class Allocator = allocator<pair<const Key, T>>>
 class multimap
@@ -368,6 +381,15 @@ public:
     iterator  erase(const_iterator first, const_iterator last);
     void clear() noexcept;
 
+    template<class C2>
+      void merge(multimap<Key, T, C2, Allocator>& source);    // C++17
+    template<class C2>
+      void merge(multimap<Key, T, C2, Allocator>&& source);   // C++17
+    template<class C2>
+      void merge(map<Key, T, C2, Allocator>& source);         // C++17
+    template<class C2>
+      void merge(map<Key, T, C2, Allocator>&& source);        // C++17
+
     void swap(multimap& m)
         noexcept(allocator_traits<allocator_type>::is_always_equal::value &&
             is_nothrow_swappable<key_compare>::value); // C++17
@@ -447,6 +469,9 @@ swap(multimap<Key, T, Compare, Allocator>& x,
      multimap<Key, T, Compare, Allocator>& y)
     noexcept(noexcept(x.swap(y)));
 
+template <class Key, class T, class Compare, class Allocator, class Predicate>
+  void erase_if(multimap<Key, T, Compare, Allocator>& c, Predicate pred);  // C++20
+
 }  // std
 
 */
@@ -460,6 +485,7 @@ swap(multimap<Key, T, Compare, Allocator>& x,
 #include <functional>
 #include <initializer_list>
 #include <type_traits>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
@@ -467,7 +493,8 @@ swap(multimap<Key, T, Compare, Allocator>& x,
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <class _Key, class _CP, class _Compare, bool _IsSmall>
+template <class _Key, class _CP, class _Compare,
+          bool = is_empty<_Compare>::value && !__libcpp_is_final<_Compare>::value>
 class __map_value_compare
     : private _Compare
 {
@@ -881,6 +908,7 @@ public:
     typedef value_type&                              reference;
     typedef const value_type&                        const_reference;
 
+    static_assert(sizeof(__diagnose_non_const_comparator<_Key, _Compare>()), "");
     static_assert((is_same<typename allocator_type::value_type, value_type>::value),
                   "Allocator::value_type must be same type as value_type");
 
@@ -925,6 +953,11 @@ public:
     typedef __insert_return_type<iterator, node_type> insert_return_type;
 #endif
 
+    template <class _Key2, class _Value2, class _Comp2, class _Alloc2>
+        friend class _LIBCPP_TEMPLATE_VIS map;
+    template <class _Key2, class _Value2, class _Comp2, class _Alloc2>
+        friend class _LIBCPP_TEMPLATE_VIS multimap;
+
     _LIBCPP_INLINE_VISIBILITY
     map()
         _NOEXCEPT_(
@@ -1299,6 +1332,38 @@ public:
     {
         return __tree_.template __node_handle_extract<node_type>(__it.__i_);
     }
+    template <class _Compare2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(map<key_type, mapped_type, _Compare2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_unique(__source.__tree_);
+    }
+    template <class _Compare2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(map<key_type, mapped_type, _Compare2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_unique(__source.__tree_);
+    }
+    template <class _Compare2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(multimap<key_type, mapped_type, _Compare2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_unique(__source.__tree_);
+    }
+    template <class _Compare2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(multimap<key_type, mapped_type, _Compare2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_unique(__source.__tree_);
+    }
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
@@ -1556,6 +1621,14 @@ swap(map<_Key, _Tp, _Compare, _Allocator>& __x,
     __x.swap(__y);
 }
 
+#if _LIBCPP_STD_VER > 17
+template <class _Key, class _Tp, class _Compare, class _Allocator, class _Predicate>
+inline _LIBCPP_INLINE_VISIBILITY
+void erase_if(map<_Key, _Tp, _Compare, _Allocator>& __c, _Predicate __pred)
+{ __libcpp_erase_if_container(__c, __pred); }
+#endif
+
+
 template <class _Key, class _Tp, class _Compare = less<_Key>,
           class _Allocator = allocator<pair<const _Key, _Tp> > >
 class _LIBCPP_TEMPLATE_VIS multimap
@@ -1570,6 +1643,7 @@ public:
     typedef value_type&                              reference;
     typedef const value_type&                        const_reference;
 
+    static_assert(sizeof(__diagnose_non_const_comparator<_Key, _Compare>()), "");
     static_assert((is_same<typename allocator_type::value_type, value_type>::value),
                   "Allocator::value_type must be same type as value_type");
 
@@ -1614,6 +1688,11 @@ public:
     typedef __map_node_handle<typename __base::__node, allocator_type> node_type;
 #endif
 
+    template <class _Key2, class _Value2, class _Comp2, class _Alloc2>
+        friend class _LIBCPP_TEMPLATE_VIS map;
+    template <class _Key2, class _Value2, class _Comp2, class _Alloc2>
+        friend class _LIBCPP_TEMPLATE_VIS multimap;
+
     _LIBCPP_INLINE_VISIBILITY
     multimap()
         _NOEXCEPT_(
@@ -1881,10 +1960,42 @@ public:
         return __tree_.template __node_handle_extract<node_type>(
             __it.__i_);
     }
+    template <class _Compare2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(multimap<key_type, mapped_type, _Compare2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __tree_.__node_handle_merge_multi(__source.__tree_);
+    }
+    template <class _Compare2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(multimap<key_type, mapped_type, _Compare2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __tree_.__node_handle_merge_multi(__source.__tree_);
+    }
+    template <class _Compare2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(map<key_type, mapped_type, _Compare2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __tree_.__node_handle_merge_multi(__source.__tree_);
+    }
+    template <class _Compare2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(map<key_type, mapped_type, _Compare2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __tree_.__node_handle_merge_multi(__source.__tree_);
+    }
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
-    void clear() {__tree_.clear();}
+    void clear() _NOEXCEPT {__tree_.clear();}
 
     _LIBCPP_INLINE_VISIBILITY
     void swap(multimap& __m)
@@ -2055,6 +2166,13 @@ swap(multimap<_Key, _Tp, _Compare, _Allocator>& __x,
     __x.swap(__y);
 }
 
+#if _LIBCPP_STD_VER > 17
+template <class _Key, class _Tp, class _Compare, class _Allocator, class _Predicate>
+inline _LIBCPP_INLINE_VISIBILITY
+void erase_if(multimap<_Key, _Tp, _Compare, _Allocator>& __c, _Predicate __pred)
+{ __libcpp_erase_if_container(__c, __pred); }
+#endif
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif  // _LIBCPP_MAP
diff --git a/include/memory b/include/memory
index a4bf89b495ad..ce2c35766233 100644
--- a/include/memory
+++ b/include/memory
@@ -43,7 +43,7 @@ struct pointer_traits<T*>
 
     template <class U> using rebind = U*;
 
-    static pointer pointer_to(<details>) noexcept;
+    static pointer pointer_to(<details>) noexcept; // constexpr in C++20
 };
 
 template <class T> constexpr T* to_address(T* p) noexcept; // C++20
@@ -212,10 +212,10 @@ template <class ForwardIterator>
 template <class ForwardIterator, class Size>
  ForwardIterator uninitialized_default_construct_n(ForwardIterator first, Size n);
 
-template <class Y> struct auto_ptr_ref {};      // removed in C++17
+template <class Y> struct auto_ptr_ref {};      // deprecated in C++11, removed in C++17
 
 template<class X>
-class auto_ptr                                  // removed in C++17
+class auto_ptr                                  // deprecated in C++11, removed in C++17
 {
 public:
     typedef X element_type;
@@ -667,6 +667,7 @@ void* align(size_t alignment, size_t size, void*& ptr, size_t& space);
 #if !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
 #  include <atomic>
 #endif
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
@@ -983,7 +984,7 @@ struct _LIBCPP_TEMPLATE_VIS pointer_traits<_Tp*>
 private:
     struct __nat {};
 public:
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static pointer pointer_to(typename conditional<is_void<element_type>::value,
                                       __nat, element_type>::type& __r) _NOEXCEPT
         {return _VSTD::addressof(__r);}
@@ -1459,29 +1460,21 @@ struct __has_select_on_container_copy_construction
 
 #else  // _LIBCPP_CXX03_LANG
 
-#ifndef _LIBCPP_HAS_NO_VARIADICS
-
-template <class _Alloc, class _Pointer, class ..._Args>
-struct __has_construct
-    : false_type
-{
-};
-
-#else  // _LIBCPP_HAS_NO_VARIADICS
+template <class _Alloc, class _Pointer, class _Tp, class = void>
+struct __has_construct : std::false_type {};
 
-template <class _Alloc, class _Pointer, class _Args>
-struct __has_construct
-    : false_type
-{
-};
+template <class _Alloc, class _Pointer, class _Tp>
+struct __has_construct<_Alloc, _Pointer, _Tp, typename __void_t<
+    decltype(_VSTD::declval<_Alloc>().construct(_VSTD::declval<_Pointer>(), _VSTD::declval<_Tp>()))
+>::type> : std::true_type {};
 
-#endif  // _LIBCPP_HAS_NO_VARIADICS
+template <class _Alloc, class _Pointer, class = void>
+struct __has_destroy : false_type {};
 
 template <class _Alloc, class _Pointer>
-struct __has_destroy
-    : false_type
-{
-};
+struct __has_destroy<_Alloc, _Pointer, typename __void_t<
+    decltype(_VSTD::declval<_Alloc>().destroy(_VSTD::declval<_Pointer>()))
+>::type> : std::true_type {};
 
 template <class _Alloc>
 struct __has_max_size
@@ -1509,6 +1502,12 @@ struct __alloc_traits_difference_type<_Alloc, _Ptr, true>
     typedef typename _Alloc::difference_type type;
 };
 
+template <class _Tp>
+struct __is_default_allocator : false_type {};
+
+template <class _Tp>
+struct __is_default_allocator<_VSTD::allocator<_Tp> > : true_type {};
+
 template <class _Alloc>
 struct _LIBCPP_TEMPLATE_VIS allocator_traits
 {
@@ -1570,9 +1569,10 @@ struct _LIBCPP_TEMPLATE_VIS allocator_traits
             }
     template <class _Tp, class _A0>
         _LIBCPP_INLINE_VISIBILITY
-        static void construct(allocator_type&, _Tp* __p, const _A0& __a0)
+        static void construct(allocator_type& __a, _Tp* __p, const _A0& __a0)
             {
-                ::new ((void*)__p) _Tp(__a0);
+                __construct(__has_construct<allocator_type, _Tp*, const _A0&>(),
+                            __a, __p, __a0);
             }
     template <class _Tp, class _A0, class _A1>
         _LIBCPP_INLINE_VISIBILITY
@@ -1612,7 +1612,7 @@ struct _LIBCPP_TEMPLATE_VIS allocator_traits
         void
         __construct_forward(allocator_type& __a, _Ptr __begin1, _Ptr __end1, _Ptr& __begin2)
         {
-            for (; __begin1 != __end1; ++__begin1, ++__begin2)
+            for (; __begin1 != __end1; ++__begin1, (void) ++__begin2)
                 construct(__a, _VSTD::__to_raw_pointer(__begin2), _VSTD::move_if_noexcept(*__begin1));
         }
 
@@ -1621,7 +1621,7 @@ struct _LIBCPP_TEMPLATE_VIS allocator_traits
         static
         typename enable_if
         <
-            (is_same<allocator_type, allocator<_Tp> >::value
+            (__is_default_allocator<allocator_type>::value
                 || !__has_construct<allocator_type, _Tp*, _Tp>::value) &&
              is_trivially_move_constructible<_Tp>::value,
             void
@@ -1646,23 +1646,25 @@ struct _LIBCPP_TEMPLATE_VIS allocator_traits
                 construct(__a, _VSTD::__to_raw_pointer(__begin2), *__begin1);
         }
 
-    template <class _Tp>
+    template <class _SourceTp, class _DestTp,
+              class _RawSourceTp = typename remove_const<_SourceTp>::type,
+              class _RawDestTp = typename remove_const<_DestTp>::type>
         _LIBCPP_INLINE_VISIBILITY
         static
         typename enable_if
         <
-            (is_same<allocator_type, allocator<_Tp> >::value
-                || !__has_construct<allocator_type, _Tp*, _Tp>::value) &&
-             is_trivially_move_constructible<_Tp>::value,
+            is_trivially_move_constructible<_DestTp>::value &&
+            is_same<_RawSourceTp, _RawDestTp>::value &&
+            (__is_default_allocator<allocator_type>::value ||
+             !__has_construct<allocator_type, _DestTp*, _SourceTp&>::value),
             void
         >::type
-        __construct_range_forward(allocator_type&, _Tp* __begin1, _Tp* __end1, _Tp*& __begin2)
+        __construct_range_forward(allocator_type&, _SourceTp* __begin1, _SourceTp* __end1, _DestTp*& __begin2)
         {
-            typedef typename remove_const<_Tp>::type _Vp;
             ptrdiff_t _Np = __end1 - __begin1;
             if (_Np > 0)
             {
-                _VSTD::memcpy(const_cast<_Vp*>(__begin2), __begin1, _Np * sizeof(_Tp));
+                _VSTD::memcpy(const_cast<_RawDestTp*>(__begin2), __begin1, _Np * sizeof(_DestTp));
                 __begin2 += _Np;
             }
         }
@@ -1685,7 +1687,7 @@ struct _LIBCPP_TEMPLATE_VIS allocator_traits
         static
         typename enable_if
         <
-            (is_same<allocator_type, allocator<_Tp> >::value
+            (__is_default_allocator<allocator_type>::value
                 || !__has_construct<allocator_type, _Tp*, _Tp>::value) &&
              is_trivially_move_constructible<_Tp>::value,
             void
@@ -1720,6 +1722,19 @@ private:
             {
                 ::new ((void*)__p) _Tp(_VSTD::forward<_Args>(__args)...);
             }
+#else  // _LIBCPP_HAS_NO_VARIADICS
+    template <class _Tp, class _A0>
+        _LIBCPP_INLINE_VISIBILITY
+        static void __construct(true_type, allocator_type& __a, _Tp* __p,
+                                const _A0& __a0)
+            {__a.construct(__p, __a0);}
+    template <class _Tp, class _A0>
+        _LIBCPP_INLINE_VISIBILITY
+        static void __construct(false_type, allocator_type&, _Tp* __p,
+                                const _A0& __a0)
+            {
+                ::new ((void*)__p) _Tp(__a0);
+            }
 #endif  // _LIBCPP_HAS_NO_VARIADICS
 
     template <class _Tp>
@@ -1782,7 +1797,7 @@ public:
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     allocator() _NOEXCEPT {}
 
-    template <class _Up> 
+    template <class _Up>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     allocator(const allocator<_Up>&) _NOEXCEPT {}
 
@@ -1790,16 +1805,16 @@ public:
         {return _VSTD::addressof(__x);}
     _LIBCPP_INLINE_VISIBILITY const_pointer address(const_reference __x) const _NOEXCEPT
         {return _VSTD::addressof(__x);}
-    _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY 
+    _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY
     pointer allocate(size_type __n, allocator<void>::const_pointer = 0)
         {
         if (__n > max_size())
             __throw_length_error("allocator<T>::allocate(size_t n)"
                                  " 'n' exceeds maximum supported size");
-        return static_cast<pointer>(_VSTD::__libcpp_allocate(__n * sizeof(_Tp), __alignof(_Tp)));
+        return static_cast<pointer>(_VSTD::__libcpp_allocate(__n * sizeof(_Tp), _LIBCPP_ALIGNOF(_Tp)));
         }
-    _LIBCPP_INLINE_VISIBILITY void deallocate(pointer __p, size_type) _NOEXCEPT
-        {_VSTD::__libcpp_deallocate((void*)__p, __alignof(_Tp));}
+    _LIBCPP_INLINE_VISIBILITY void deallocate(pointer __p, size_type __n) _NOEXCEPT
+        {_VSTD::__libcpp_deallocate((void*)__p, __n * sizeof(_Tp), _LIBCPP_ALIGNOF(_Tp));}
     _LIBCPP_INLINE_VISIBILITY size_type max_size() const _NOEXCEPT
         {return size_type(~0) / sizeof(_Tp);}
 #if !defined(_LIBCPP_HAS_NO_RVALUE_REFERENCES) && !defined(_LIBCPP_HAS_NO_VARIADICS)
@@ -1887,7 +1902,7 @@ public:
     allocator() _NOEXCEPT {}
 
     template <class _Up>
-    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     allocator(const allocator<_Up>&) _NOEXCEPT {}
 
     _LIBCPP_INLINE_VISIBILITY const_pointer address(const_reference __x) const _NOEXCEPT
@@ -1897,10 +1912,10 @@ public:
         if (__n > max_size())
             __throw_length_error("allocator<const T>::allocate(size_t n)"
                                  " 'n' exceeds maximum supported size");
-        return static_cast<pointer>(_VSTD::__libcpp_allocate(__n * sizeof(_Tp), __alignof(_Tp)));
+        return static_cast<pointer>(_VSTD::__libcpp_allocate(__n * sizeof(_Tp), _LIBCPP_ALIGNOF(_Tp)));
     }
-    _LIBCPP_INLINE_VISIBILITY void deallocate(pointer __p, size_type) _NOEXCEPT
-        {_VSTD::__libcpp_deallocate((void*) const_cast<_Tp *>(__p), __alignof(_Tp));}
+    _LIBCPP_INLINE_VISIBILITY void deallocate(pointer __p, size_type __n) _NOEXCEPT
+        {_VSTD::__libcpp_deallocate((void*) const_cast<_Tp *>(__p), __n * sizeof(_Tp), _LIBCPP_ALIGNOF(_Tp));}
     _LIBCPP_INLINE_VISIBILITY size_type max_size() const _NOEXCEPT
         {return size_type(~0) / sizeof(_Tp);}
 #if !defined(_LIBCPP_HAS_NO_RVALUE_REFERENCES) && !defined(_LIBCPP_HAS_NO_VARIADICS)
@@ -1989,10 +2004,10 @@ public:
     _LIBCPP_INLINE_VISIBILITY explicit raw_storage_iterator(_OutputIterator __x) : __x_(__x) {}
     _LIBCPP_INLINE_VISIBILITY raw_storage_iterator& operator*() {return *this;}
     _LIBCPP_INLINE_VISIBILITY raw_storage_iterator& operator=(const _Tp& __element)
-        {::new(&*__x_) _Tp(__element); return *this;}
+        {::new(_VSTD::addressof(*__x_)) _Tp(__element); return *this;}
 #if _LIBCPP_STD_VER >= 14
     _LIBCPP_INLINE_VISIBILITY raw_storage_iterator& operator=(_Tp&& __element)
-        {::new(&*__x_) _Tp(_VSTD::move(__element)); return *this;}
+        {::new(_VSTD::addressof(*__x_)) _Tp(_VSTD::move(__element)); return *this;}
 #endif
     _LIBCPP_INLINE_VISIBILITY raw_storage_iterator& operator++() {++__x_; return *this;}
     _LIBCPP_INLINE_VISIBILITY raw_storage_iterator  operator++(int)
@@ -2003,7 +2018,7 @@ public:
 };
 
 template <class _Tp>
-_LIBCPP_NO_CFI
+_LIBCPP_NODISCARD_EXT _LIBCPP_NO_CFI
 pair<_Tp*, ptrdiff_t>
 get_temporary_buffer(ptrdiff_t __n) _NOEXCEPT
 {
@@ -2016,7 +2031,7 @@ get_temporary_buffer(ptrdiff_t __n) _NOEXCEPT
     while (__n > 0)
     {
 #if !defined(_LIBCPP_HAS_NO_ALIGNED_ALLOCATION)
-    if (__is_overaligned_for_new(__alignof(_Tp)))
+    if (__is_overaligned_for_new(_LIBCPP_ALIGNOF(_Tp)))
         {
             std::align_val_t __al =
                 std::align_val_t(std::alignment_of<_Tp>::value);
@@ -2027,7 +2042,7 @@ get_temporary_buffer(ptrdiff_t __n) _NOEXCEPT
                 __n * sizeof(_Tp), nothrow));
         }
 #else
-    if (__is_overaligned_for_new(__alignof(_Tp)))
+    if (__is_overaligned_for_new(_LIBCPP_ALIGNOF(_Tp)))
         {
             // Since aligned operator new is unavailable, return an empty
             // buffer rather than one with invalid alignment.
@@ -2051,18 +2066,18 @@ template <class _Tp>
 inline _LIBCPP_INLINE_VISIBILITY
 void return_temporary_buffer(_Tp* __p) _NOEXCEPT
 {
-  _VSTD::__libcpp_deallocate((void*)__p, __alignof(_Tp));
+  _VSTD::__libcpp_deallocate_unsized((void*)__p, _LIBCPP_ALIGNOF(_Tp));
 }
 
 #if _LIBCPP_STD_VER <= 14 || defined(_LIBCPP_ENABLE_CXX17_REMOVED_AUTO_PTR)
 template <class _Tp>
-struct auto_ptr_ref
+struct _LIBCPP_DEPRECATED_IN_CXX11 auto_ptr_ref
 {
     _Tp* __ptr_;
 };
 
 template<class _Tp>
-class _LIBCPP_TEMPLATE_VIS auto_ptr
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_DEPRECATED_IN_CXX11 auto_ptr
 {
 private:
     _Tp* __ptr_;
@@ -2106,7 +2121,7 @@ public:
 };
 
 template <>
-class _LIBCPP_TEMPLATE_VIS auto_ptr<void>
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_DEPRECATED_IN_CXX11 auto_ptr<void>
 {
 public:
     typedef void element_type;
@@ -2130,7 +2145,9 @@ struct __compressed_pair_elem {
   _LIBCPP_INLINE_VISIBILITY
   constexpr explicit
   __compressed_pair_elem(_Up&& __u)
-      : __value_(_VSTD::forward<_Up>(__u)){};
+      : __value_(_VSTD::forward<_Up>(__u))
+    {
+    }
 
   template <class... _Args, size_t... _Indexes>
   _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
@@ -2167,7 +2184,8 @@ struct __compressed_pair_elem<_Tp, _Idx, true> : private _Tp {
   _LIBCPP_INLINE_VISIBILITY
   constexpr explicit
   __compressed_pair_elem(_Up&& __u)
-      : __value_type(_VSTD::forward<_Up>(__u)){};
+      : __value_type(_VSTD::forward<_Up>(__u))
+  {}
 
   template <class... _Args, size_t... _Indexes>
   _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
@@ -3682,7 +3700,7 @@ private:
     virtual void __on_zero_shared_weak() _NOEXCEPT;
 public:
     _LIBCPP_INLINE_VISIBILITY
-    _Tp* get() _NOEXCEPT {return &__data_.second();}
+    _Tp* get() _NOEXCEPT {return _VSTD::addressof(__data_.second());}
 };
 
 template <class _Tp, class _Alloc>
@@ -5624,15 +5642,18 @@ template <class _Tp, class _Alloc>
 struct __temp_value {
     typedef allocator_traits<_Alloc> _Traits;
 
-    typename aligned_storage<sizeof(_Tp), alignof(_Tp)>::type __v;
+    typename aligned_storage<sizeof(_Tp), _LIBCPP_ALIGNOF(_Tp)>::type __v;
     _Alloc &__a;
 
     _Tp *__addr() { return reinterpret_cast<_Tp *>(addressof(__v)); }
     _Tp &   get() { return *__addr(); }
 
     template<class... _Args>
-    __temp_value(_Alloc &__alloc, _Args&& ... __args) : __a(__alloc)
-    { _Traits::construct(__a, __addr(), _VSTD::forward<_Args>(__args)...); }
+    _LIBCPP_NO_CFI
+    __temp_value(_Alloc &__alloc, _Args&& ... __args) : __a(__alloc) {
+      _Traits::construct(__a, reinterpret_cast<_Tp*>(addressof(__v)),
+                         _VSTD::forward<_Args>(__args)...);
+    }
 
     ~__temp_value() { _Traits::destroy(__a, __addr()); }
     };
diff --git a/include/module.modulemap b/include/module.modulemap
index 089505586fb3..6d88f52113cb 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -228,6 +228,10 @@ module std [system] {
     header "atomic"
     export *
   }
+  module bit {
+    header "bit"
+    export *
+  }
   module bitset {
     header "bitset"
     export string
@@ -520,10 +524,6 @@ module std [system] {
       header "experimental/deque"
       export *
     }
-    module dynarray {
-      header "experimental/dynarray"
-      export *
-    }
     module filesystem {
       header "experimental/filesystem"
       export *
diff --git a/include/mutex b/include/mutex
index 52e39b0fb1f8..6d2de2b460e2 100644
--- a/include/mutex
+++ b/include/mutex
@@ -194,6 +194,7 @@ template<class Callable, class ...Args>
 #ifndef _LIBCPP_CXX03_LANG
 #include <tuple>
 #endif
+#include <version>
 #include <__threading_support>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -488,7 +489,7 @@ public:
 };
 
 template <class _Mutex>
-class _LIBCPP_TEMPLATE_VIS scoped_lock<_Mutex> {
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_THREAD_SAFETY_ANNOTATION(scoped_lockable) scoped_lock<_Mutex> {
 public:
     typedef _Mutex  mutex_type;
 private:
diff --git a/include/new b/include/new
index e70b9c621570..24ffcad5a6bc 100644
--- a/include/new
+++ b/include/new
@@ -27,12 +27,6 @@ public:
     virtual const char* what() const noexcept;
 };
 
-class bad_array_length : public bad_alloc // FIXME: Not part of C++
-{
-public:
-    bad_array_length() noexcept;
-};
-
 class bad_array_new_length : public bad_alloc // C++14
 {
 public:
@@ -91,6 +85,7 @@ void  operator delete[](void* ptr, void*) noexcept;
 #include <exception>
 #include <type_traits>
 #include <cstddef>
+#include <version>
 #ifdef _LIBCPP_NO_EXCEPTIONS
 #include <cstdlib>
 #endif
@@ -103,23 +98,23 @@ void  operator delete[](void* ptr, void*) noexcept;
 #pragma GCC system_header
 #endif
 
-#if !(defined(_LIBCPP_BUILDING_LIBRARY) || _LIBCPP_STD_VER >= 14 || \
-    (defined(__cpp_sized_deallocation) && __cpp_sized_deallocation >= 201309))
-# define _LIBCPP_HAS_NO_SIZED_DEALLOCATION
+#if !defined(__cpp_sized_deallocation) || __cpp_sized_deallocation  < 201309L
+#define _LIBCPP_HAS_NO_LANGUAGE_SIZED_DEALLOCATION
 #endif
 
-#if !defined(_LIBCPP_HAS_NO_ALIGNED_ALLOCATION) && \
-    (!(defined(_LIBCPP_BUILDING_LIBRARY) || _LIBCPP_STD_VER > 14 || \
-    (defined(__cpp_aligned_new) && __cpp_aligned_new >= 201606)))
-# define _LIBCPP_HAS_NO_ALIGNED_ALLOCATION
+#if !defined(_LIBCPP_BUILDING_LIBRARY) && _LIBCPP_STD_VER < 14 && \
+    defined(_LIBCPP_HAS_NO_LANGUAGE_SIZED_DEALLOCATION)
+# define _LIBCPP_HAS_NO_LIBRARY_SIZED_DEALLOCATION
 #endif
 
+#if defined(_LIBCPP_HAS_NO_LIBRARY_SIZED_DEALLOCATION) || \
+    defined(_LIBCPP_HAS_NO_LANGUAGE_SIZED_DEALLOCATION)
+# define _LIBCPP_HAS_NO_SIZED_DEALLOCATION
+#endif
 
 #if !__has_builtin(__builtin_operator_new) || \
-   __has_builtin(__builtin_operator_new) < 201802L || \
-   defined(_LIBCPP_HAS_NO_ALIGNED_ALLOCATION) || \
-   !defined(__cpp_aligned_new) || __cpp_aligned_new < 201606
-#define _LIBCPP_HAS_NO_BUILTIN_ALIGNED_OPERATOR_NEW_DELETE
+   __has_builtin(__builtin_operator_new) < 201802L
+#define _LIBCPP_HAS_NO_BUILTIN_OVERLOADED_OPERATOR_NEW_DELETE
 #endif
 
 namespace std  // purposefully not using versioning namespace
@@ -155,29 +150,14 @@ _LIBCPP_FUNC_VIS new_handler get_new_handler() _NOEXCEPT;
 
 _LIBCPP_NORETURN _LIBCPP_FUNC_VIS void __throw_bad_alloc();  // not in C++ spec
 
-#if defined(_LIBCPP_BUILDING_LIBRARY) || (_LIBCPP_STD_VER > 11)
-
-class _LIBCPP_EXCEPTION_ABI _LIBCPP_AVAILABILITY_BAD_ARRAY_LENGTH
-    bad_array_length : public bad_alloc {
-public:
-    bad_array_length() _NOEXCEPT;
-    virtual ~bad_array_length() _NOEXCEPT;
-    virtual const char* what() const _NOEXCEPT;
-};
-
-#define _LIBCPP_BAD_ARRAY_LENGTH_DEFINED
-
-#endif  // defined(_LIBCPP_BUILDING_LIBRARY) || (_LIBCPP_STD_VER > 11)
-
-#if !defined(_LIBCPP_ABI_MICROSOFT) || defined(_LIBCPP_NO_VCRUNTIME)
-#if !defined(_LIBCPP_HAS_NO_ALIGNED_ALLOCATION) || _LIBCPP_STD_VER > 14
+#if !defined(_LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION) && \
+    !defined(_LIBCPP_DEFER_NEW_TO_VCRUNTIME)
 #ifndef _LIBCPP_CXX03_LANG
 enum class _LIBCPP_ENUM_VIS align_val_t : size_t { };
 #else
 enum align_val_t { __zero = 0, __max = (size_t)-1 };
 #endif
 #endif
-#endif
 
 }  // std
 
@@ -187,13 +167,13 @@ enum align_val_t { __zero = 0, __max = (size_t)-1 };
 #define _THROW_BAD_ALLOC
 #endif
 
-#if !defined(_LIBCPP_ABI_MICROSOFT) || defined(_LIBCPP_NO_VCRUNTIME)
+#if !defined(_LIBCPP_DEFER_NEW_TO_VCRUNTIME)
 
 _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_OVERRIDABLE_FUNC_VIS void* operator new(std::size_t __sz) _THROW_BAD_ALLOC;
 _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_OVERRIDABLE_FUNC_VIS void* operator new(std::size_t __sz, const std::nothrow_t&) _NOEXCEPT _NOALIAS;
 _LIBCPP_OVERRIDABLE_FUNC_VIS void  operator delete(void* __p) _NOEXCEPT;
 _LIBCPP_OVERRIDABLE_FUNC_VIS void  operator delete(void* __p, const std::nothrow_t&) _NOEXCEPT;
-#ifndef _LIBCPP_HAS_NO_SIZED_DEALLOCATION
+#ifndef _LIBCPP_HAS_NO_LIBRARY_SIZED_DEALLOCATION
 _LIBCPP_OVERRIDABLE_FUNC_VIS _LIBCPP_AVAILABILITY_SIZED_NEW_DELETE void  operator delete(void* __p, std::size_t __sz) _NOEXCEPT;
 #endif
 
@@ -201,16 +181,16 @@ _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_OVERRIDABLE_FUNC_VIS void* operator new[](
 _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_OVERRIDABLE_FUNC_VIS void* operator new[](std::size_t __sz, const std::nothrow_t&) _NOEXCEPT _NOALIAS;
 _LIBCPP_OVERRIDABLE_FUNC_VIS void  operator delete[](void* __p) _NOEXCEPT;
 _LIBCPP_OVERRIDABLE_FUNC_VIS void  operator delete[](void* __p, const std::nothrow_t&) _NOEXCEPT;
-#ifndef _LIBCPP_HAS_NO_SIZED_DEALLOCATION
+#ifndef _LIBCPP_HAS_NO_LIBRARY_SIZED_DEALLOCATION
 _LIBCPP_OVERRIDABLE_FUNC_VIS _LIBCPP_AVAILABILITY_SIZED_NEW_DELETE void  operator delete[](void* __p, std::size_t __sz) _NOEXCEPT;
 #endif
 
-#ifndef _LIBCPP_HAS_NO_ALIGNED_ALLOCATION
+#ifndef _LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION
 _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_OVERRIDABLE_FUNC_VIS void* operator new(std::size_t __sz, std::align_val_t) _THROW_BAD_ALLOC;
 _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_OVERRIDABLE_FUNC_VIS void* operator new(std::size_t __sz, std::align_val_t, const std::nothrow_t&) _NOEXCEPT _NOALIAS;
 _LIBCPP_OVERRIDABLE_FUNC_VIS void  operator delete(void* __p, std::align_val_t) _NOEXCEPT;
 _LIBCPP_OVERRIDABLE_FUNC_VIS void  operator delete(void* __p, std::align_val_t, const std::nothrow_t&) _NOEXCEPT;
-#ifndef _LIBCPP_HAS_NO_SIZED_DEALLOCATION
+#ifndef _LIBCPP_HAS_NO_LIBRARY_SIZED_DEALLOCATION
 _LIBCPP_OVERRIDABLE_FUNC_VIS _LIBCPP_AVAILABILITY_SIZED_NEW_DELETE void  operator delete(void* __p, std::size_t __sz, std::align_val_t) _NOEXCEPT;
 #endif
 
@@ -218,7 +198,7 @@ _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_OVERRIDABLE_FUNC_VIS void* operator new[](
 _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_OVERRIDABLE_FUNC_VIS void* operator new[](std::size_t __sz, std::align_val_t, const std::nothrow_t&) _NOEXCEPT _NOALIAS;
 _LIBCPP_OVERRIDABLE_FUNC_VIS void  operator delete[](void* __p, std::align_val_t) _NOEXCEPT;
 _LIBCPP_OVERRIDABLE_FUNC_VIS void  operator delete[](void* __p, std::align_val_t, const std::nothrow_t&) _NOEXCEPT;
-#ifndef _LIBCPP_HAS_NO_SIZED_DEALLOCATION
+#ifndef _LIBCPP_HAS_NO_LIBRARY_SIZED_DEALLOCATION
 _LIBCPP_OVERRIDABLE_FUNC_VIS _LIBCPP_AVAILABILITY_SIZED_NEW_DELETE void  operator delete[](void* __p, std::size_t __sz, std::align_val_t) _NOEXCEPT;
 #endif
 #endif
@@ -228,7 +208,7 @@ _LIBCPP_NODISCARD_AFTER_CXX17 inline _LIBCPP_INLINE_VISIBILITY void* operator ne
 inline _LIBCPP_INLINE_VISIBILITY void  operator delete  (void*, void*) _NOEXCEPT {}
 inline _LIBCPP_INLINE_VISIBILITY void  operator delete[](void*, void*) _NOEXCEPT {}
 
-#endif // !_LIBCPP_ABI_MICROSOFT || _LIBCPP_NO_VCRUNTIME
+#endif // !_LIBCPP_DEFER_NEW_TO_VCRUNTIME
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
@@ -244,7 +224,7 @@ inline _LIBCPP_INLINE_VISIBILITY void *__libcpp_allocate(size_t __size, size_t _
 #ifndef _LIBCPP_HAS_NO_ALIGNED_ALLOCATION
   if (__is_overaligned_for_new(__align)) {
     const align_val_t __align_val = static_cast<align_val_t>(__align);
-# ifdef _LIBCPP_HAS_NO_BUILTIN_ALIGNED_OPERATOR_NEW_DELETE
+# ifdef _LIBCPP_HAS_NO_BUILTIN_OVERLOADED_OPERATOR_NEW_DELETE
     return ::operator new(__size, __align_val);
 # else
     return __builtin_operator_new(__size, __align_val);
@@ -260,43 +240,98 @@ inline _LIBCPP_INLINE_VISIBILITY void *__libcpp_allocate(size_t __size, size_t _
 #endif
 }
 
-inline _LIBCPP_INLINE_VISIBILITY void __libcpp_deallocate(void* __ptr, size_t __align) {
-#ifndef _LIBCPP_HAS_NO_ALIGNED_ALLOCATION
-  if (__is_overaligned_for_new(__align)) {
-    const align_val_t __align_val = static_cast<align_val_t>(__align);
-# ifdef _LIBCPP_HAS_NO_BUILTIN_ALIGNED_OPERATOR_NEW_DELETE
-    return ::operator delete(__ptr, __align_val);
-# else
-    return __builtin_operator_delete(__ptr, __align_val);
-# endif
+struct _DeallocateCaller {
+  static inline _LIBCPP_INLINE_VISIBILITY
+  void __do_deallocate_handle_size_align(void *__ptr, size_t __size, size_t __align) {
+#if defined(_LIBCPP_HAS_NO_ALIGNED_ALLOCATION)
+    ((void)__align);
+    return __do_deallocate_handle_size(__ptr, __size);
+#else
+    if (__is_overaligned_for_new(__align)) {
+      const align_val_t __align_val = static_cast<align_val_t>(__align);
+      return __do_deallocate_handle_size(__ptr, __size, __align_val);
+    } else {
+      return __do_deallocate_handle_size(__ptr, __size);
+    }
+#endif
+  }
+
+  static inline _LIBCPP_INLINE_VISIBILITY
+  void __do_deallocate_handle_align(void *__ptr, size_t __align) {
+#if defined(_LIBCPP_HAS_NO_ALIGNED_ALLOCATION)
+    ((void)__align);
+    return __do_call(__ptr);
+#else
+    if (__is_overaligned_for_new(__align)) {
+      const align_val_t __align_val = static_cast<align_val_t>(__align);
+      return __do_call(__ptr, __align_val);
+    } else {
+      return __do_call(__ptr);
+    }
+#endif
   }
+
+ private:
+  static inline void __do_deallocate_handle_size(void *__ptr, size_t __size) {
+#ifdef _LIBCPP_HAS_NO_SIZED_DEALLOCATION
+    ((void)__size);
+    return __do_call(__ptr);
 #else
-  ((void)__align);
+    return __do_call(__ptr, __size);
 #endif
-#ifdef _LIBCPP_HAS_NO_BUILTIN_OPERATOR_NEW_DELETE
-  return ::operator delete(__ptr);
+  }
+
+#ifndef _LIBCPP_HAS_NO_ALIGNED_ALLOCATION
+  static inline void __do_deallocate_handle_size(void *__ptr, size_t __size, align_val_t __align) {
+#ifdef _LIBCPP_HAS_NO_SIZED_DEALLOCATION
+    ((void)__size);
+    return __do_call(__ptr, __align);
 #else
-  return __builtin_operator_delete(__ptr);
+    return __do_call(__ptr, __size, __align);
+#endif
+  }
 #endif
-}
 
-#ifdef _LIBCPP_BAD_ARRAY_LENGTH_DEFINED
-_LIBCPP_NORETURN inline _LIBCPP_INLINE_VISIBILITY
-#ifndef _LIBCPP_NO_EXCEPTIONS
-_LIBCPP_AVAILABILITY_BAD_ARRAY_LENGTH
+private:
+  template <class _A1, class _A2>
+  static inline void __do_call(void *__ptr, _A1 __a1, _A2 __a2) {
+#if defined(_LIBCPP_HAS_NO_BUILTIN_OPERATOR_NEW_DELETE) || \
+    defined(_LIBCPP_HAS_NO_BUILTIN_OVERLOADED_OPERATOR_NEW_DELETE)
+    return ::operator delete(__ptr, __a1, __a2);
+#else
+    return __builtin_operator_delete(__ptr, __a1, __a2);
 #endif
-void __throw_bad_array_length()
-{
-#ifndef _LIBCPP_NO_EXCEPTIONS
-    throw bad_array_length();
+  }
+
+  template <class _A1>
+  static inline void __do_call(void *__ptr, _A1 __a1) {
+#if defined(_LIBCPP_HAS_NO_BUILTIN_OPERATOR_NEW_DELETE) || \
+    defined(_LIBCPP_HAS_NO_BUILTIN_OVERLOADED_OPERATOR_NEW_DELETE)
+    return ::operator delete(__ptr, __a1);
 #else
-	_VSTD::abort();
+    return __builtin_operator_delete(__ptr, __a1);
 #endif
-}
+  }
+
+  static inline void __do_call(void *__ptr) {
+#ifdef _LIBCPP_HAS_NO_BUILTIN_OPERATOR_NEW_DELETE
+    return ::operator delete(__ptr);
+#else
+    return __builtin_operator_delete(__ptr);
 #endif
+  }
+};
+
+inline _LIBCPP_INLINE_VISIBILITY void __libcpp_deallocate(void* __ptr, size_t __size, size_t __align) {
+  _DeallocateCaller::__do_deallocate_handle_size_align(__ptr, __size, __align);
+}
+
+inline _LIBCPP_INLINE_VISIBILITY void __libcpp_deallocate_unsized(void* __ptr, size_t __align) {
+  _DeallocateCaller::__do_deallocate_handle_align(__ptr, __align);
+}
 
 template <class _Tp>
-_LIBCPP_NODISCARD_AFTER_CXX17 inline 
+_LIBCPP_NODISCARD_AFTER_CXX17 inline
 _LIBCPP_CONSTEXPR _Tp* __launder(_Tp* __p) _NOEXCEPT
 {
     static_assert (!(is_function<_Tp>::value), "can't launder functions" );
diff --git a/include/numeric b/include/numeric
index b33b6a398eb0..4e68239d059e 100644
--- a/include/numeric
+++ b/include/numeric
@@ -104,15 +104,15 @@ template<class InputIterator, class OutputIterator, class T,
 
 template<class InputIterator, class OutputIterator,
          class BinaryOperation, class UnaryOperation>
-	OutputIterator
-	transform_inclusive_scan(InputIterator first, InputIterator last,
+    OutputIterator
+    transform_inclusive_scan(InputIterator first, InputIterator last,
                              OutputIterator result,
                              BinaryOperation binary_op, UnaryOperation unary_op);  // C++17
 
 template<class InputIterator, class OutputIterator,
          class BinaryOperation, class UnaryOperation, class T>
-	OutputIterator
-	transform_inclusive_scan(InputIterator first, InputIterator last,
+    OutputIterator
+    transform_inclusive_scan(InputIterator first, InputIterator last,
                              OutputIterator result,
                              BinaryOperation binary_op, UnaryOperation unary_op,
                              T init);  // C++17
@@ -142,6 +142,7 @@ template <class M, class N>
 #include <iterator>
 #include <limits> // for numeric_limits
 #include <functional>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
diff --git a/include/optional b/include/optional
index a76f8d18976e..70422068e285 100644
--- a/include/optional
+++ b/include/optional
@@ -105,8 +105,8 @@ namespace std {
 
     // 23.6.3.3, assignment
     optional &operator=(nullopt_t) noexcept;
-    optional &operator=(const optional &);
-    optional &operator=(optional &&) noexcept(see below );
+    optional &operator=(const optional &);                // constexpr in C++20
+    optional &operator=(optional &&) noexcept(see below); // constexpr in C++20
     template <class U = T> optional &operator=(U &&);
     template <class U> optional &operator=(const optional<U> &);
     template <class U> optional &operator=(optional<U> &&);
@@ -156,6 +156,7 @@ template<class T>
 #include <stdexcept>
 #include <type_traits>
 #include <utility>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
@@ -168,7 +169,7 @@ _LIBCPP_PUSH_MACROS
 namespace std  // purposefully not using versioning namespace
 {
 
-class _LIBCPP_EXCEPTION_ABI bad_optional_access
+class _LIBCPP_EXCEPTION_ABI _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS bad_optional_access
     : public exception
 {
 public:
@@ -185,6 +186,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 _LIBCPP_NORETURN
 inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_AVAILABILITY_THROW_BAD_OPTIONAL_ACCESS
 void __throw_bad_optional_access() {
 #ifndef _LIBCPP_NO_EXCEPTIONS
         throw bad_optional_access();
@@ -932,6 +934,7 @@ public:
     using __base::__get;
 
     _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_AVAILABILITY_THROW_BAD_OPTIONAL_ACCESS
     constexpr value_type const& value() const&
     {
         if (!this->has_value())
@@ -940,6 +943,7 @@ public:
     }
 
     _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_AVAILABILITY_THROW_BAD_OPTIONAL_ACCESS
     constexpr value_type& value() &
     {
         if (!this->has_value())
@@ -948,6 +952,7 @@ public:
     }
 
     _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_AVAILABILITY_THROW_BAD_OPTIONAL_ACCESS
     constexpr value_type&& value() &&
     {
         if (!this->has_value())
@@ -956,6 +961,7 @@ public:
     }
 
     _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_AVAILABILITY_THROW_BAD_OPTIONAL_ACCESS
     constexpr value_type const&& value() const&&
     {
         if (!this->has_value())
diff --git a/include/ostream b/include/ostream
index 5404e0dca6c8..d700a369b34a 100644
--- a/include/ostream
+++ b/include/ostream
@@ -140,6 +140,7 @@ template <class charT, class traits, class T>
 #include <locale>
 #include <iterator>
 #include <bitset>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
@@ -160,7 +161,7 @@ public:
     typedef typename traits_type::off_type off_type;
 
     // 27.7.2.2 Constructor/destructor:
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     explicit basic_ostream(basic_streambuf<char_type, traits_type>* __sb)
     { this->init(__sb); }
     virtual ~basic_ostream();
@@ -173,7 +174,7 @@ protected:
     inline _LIBCPP_INLINE_VISIBILITY
     basic_ostream& operator=(basic_ostream&& __rhs);
 #endif
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     void swap(basic_ostream& __rhs)
     { basic_ios<char_type, traits_type>::swap(__rhs); }
 
@@ -190,16 +191,16 @@ public:
     class _LIBCPP_TEMPLATE_VIS sentry;
 
     // 27.7.2.6 Formatted output:
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     basic_ostream& operator<<(basic_ostream& (*__pf)(basic_ostream&))
     { return __pf(*this); }
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     basic_ostream& operator<<(basic_ios<char_type, traits_type>&
                               (*__pf)(basic_ios<char_type,traits_type>&))
     { __pf(*this); return *this; }
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     basic_ostream& operator<<(ios_base& (*__pf)(ios_base&))
     { __pf(*this); return *this; }
 
@@ -224,11 +225,11 @@ public:
     basic_ostream& flush();
 
     // 27.7.2.5 seeks:
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     pos_type tellp();
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     basic_ostream& seekp(pos_type __pos);
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     basic_ostream& seekp(off_type __off, ios_base::seekdir __dir);
 
 protected:
@@ -1092,7 +1093,7 @@ operator<<(basic_ostream<_CharT, _Traits>& __os, const bitset<_Size>& __x)
                          use_facet<ctype<_CharT> >(__os.getloc()).widen('1'));
 }
 
-#ifndef _LIBCPP_AVAILABILITY_NO_STREAMS_EXTERN_TEMPLATE
+#ifndef _LIBCPP_DO_NOT_ASSUME_STREAMS_EXPLICIT_INSTANTIATION_IN_DYLIB
 _LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS basic_ostream<char>)
 _LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS basic_ostream<wchar_t>)
 #endif
diff --git a/include/random b/include/random
index 89664a6ca331..724bd0fc2154 100644
--- a/include/random
+++ b/include/random
@@ -2337,7 +2337,7 @@ mersenne_twister_engine<_UIntType, __w, __n, __m, __r, __a, __u, __d, __s, __b,
         for (size_t __i = 1; __i < __n; ++__i)
             if (__x_[__i] != 0)
                 return;
-        __x_[0] = _Max;
+        __x_[0] = result_type(1) << (__w - 1);
     }
 }
 
@@ -2363,7 +2363,7 @@ mersenne_twister_engine<_UIntType, __w, __n, __m, __r, __a, __u, __d, __s, __b,
         for (size_t __i = 1; __i < __n; ++__i)
             if (__x_[__i] != 0)
                 return;
-        __x_[0] = _Max;
+        __x_[0] = result_type(1) << (__w - 1);
     }
 }
 
diff --git a/include/regex b/include/regex
index 84aacc029edc..bd83d7c10ca7 100644
--- a/include/regex
+++ b/include/regex
@@ -769,6 +769,7 @@ typedef regex_token_iterator<wstring::const_iterator> wsregex_token_iterator;
 #include <memory>
 #include <vector>
 #include <deque>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
@@ -989,6 +990,10 @@ public:
 
 #if defined(__mips__) && defined(__GLIBC__)
     static const char_class_type __regex_word = static_cast<char_class_type>(_ISbit(15));
+#elif defined(__NetBSD__)
+    // NetBSD defines classes up to 0x2000
+    // see sys/ctype_bits.h, _CTYPE_Q
+    static const char_class_type __regex_word = 0x8000;
 #else
     static const char_class_type __regex_word = 0x80;
 #endif
@@ -1351,9 +1356,9 @@ public:
     virtual ~__node() {}
 
     _LIBCPP_INLINE_VISIBILITY
-    virtual void __exec(__state&) const {};
+    virtual void __exec(__state&) const {}
     _LIBCPP_INLINE_VISIBILITY
-    virtual void __exec_split(bool, __state&) const {};
+    virtual void __exec_split(bool, __state&) const {}
 };
 
 // __end_state
@@ -2414,20 +2419,17 @@ __bracket_expression<_CharT, _Traits>::__exec(__state& __s) const
                 goto __exit;
             }
         }
-        // set of "__found" chars =
+        // When there's at least one of __neg_chars_ and __neg_mask_, the set
+        // of "__found" chars is
         //   union(complement(union(__neg_chars_, __neg_mask_)),
         //         other cases...)
         //
-        // __neg_chars_ and __neg_mask_'d better be handled together, as there
-        // are no short circuit opportunities.
-        //
-        // In addition, when __neg_mask_/__neg_chars_ is empty, they should be
-        // treated as all ones/all chars.
+        // It doesn't make sense to check this when there are no __neg_chars_
+        // and no __neg_mask_.
+        if (!(__neg_mask_ == 0 && __neg_chars_.empty()))
         {
-          const bool __in_neg_mask = (__neg_mask_ == 0) ||
-              __traits_.isctype(__ch, __neg_mask_);
+            const bool __in_neg_mask = __traits_.isctype(__ch, __neg_mask_);
           const bool __in_neg_chars =
-              __neg_chars_.empty() ||
               std::find(__neg_chars_.begin(), __neg_chars_.end(), __ch) !=
               __neg_chars_.end();
           if (!(__in_neg_mask || __in_neg_chars))
diff --git a/include/scoped_allocator b/include/scoped_allocator
index 4760d946def7..bdbb0136b5c3 100644
--- a/include/scoped_allocator
+++ b/include/scoped_allocator
@@ -108,6 +108,7 @@ template <class OuterA1, class OuterA2, class... InnerAllocs>
 
 #include <__config>
 #include <memory>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
diff --git a/include/set b/include/set
index 108a9e97f88d..a0155f0b2758 100644
--- a/include/set
+++ b/include/set
@@ -128,6 +128,15 @@ public:
     iterator  erase(const_iterator first, const_iterator last);
     void clear() noexcept;
 
+    template<class C2>
+      void merge(set<Key, C2, Allocator>& source);         // C++17
+    template<class C2>
+      void merge(set<Key, C2, Allocator>&& source);        // C++17
+    template<class C2>
+      void merge(multiset<Key, C2, Allocator>& source);    // C++17
+    template<class C2>
+      void merge(multiset<Key, C2, Allocator>&& source);   // C++17
+
     void swap(set& s)
         noexcept(
             __is_nothrow_swappable<key_compare>::value &&
@@ -207,6 +216,9 @@ void
 swap(set<Key, Compare, Allocator>& x, set<Key, Compare, Allocator>& y)
     noexcept(noexcept(x.swap(y)));
 
+template <class Key, class Compare, class Allocator, class Predicate>
+  void erase_if(set<Key, Compare, Allocator>& c, Predicate pred);  // C++20
+
 template <class Key, class Compare = less<Key>,
           class Allocator = allocator<Key>>
 class multiset
@@ -316,6 +328,15 @@ public:
     iterator  erase(const_iterator first, const_iterator last);
     void clear() noexcept;
 
+    template<class C2>
+      void merge(multiset<Key, C2, Allocator>& source);    // C++17
+    template<class C2>
+      void merge(multiset<Key, C2, Allocator>&& source);   // C++17
+    template<class C2>
+      void merge(set<Key, C2, Allocator>& source);         // C++17
+    template<class C2>
+      void merge(set<Key, C2, Allocator>&& source);        // C++17
+
     void swap(multiset& s)
         noexcept(
             __is_nothrow_swappable<key_compare>::value &&
@@ -394,6 +415,9 @@ void
 swap(multiset<Key, Compare, Allocator>& x, multiset<Key, Compare, Allocator>& y)
     noexcept(noexcept(x.swap(y)));
 
+template <class Key, class Compare, class Allocator, class Predicate>
+  void erase_if(multiset<Key, Compare, Allocator>& c, Predicate pred);  // C++20
+
 }  // std
 
 */
@@ -402,6 +426,7 @@ swap(multiset<Key, Compare, Allocator>& x, multiset<Key, Compare, Allocator>& y)
 #include <__tree>
 #include <__node_handle>
 #include <functional>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
@@ -409,6 +434,9 @@ swap(multiset<Key, Compare, Allocator>& x, multiset<Key, Compare, Allocator>& y)
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+template <class _Key, class _Compare, class _Allocator>
+class multiset;
+
 template <class _Key, class _Compare = less<_Key>,
           class _Allocator = allocator<_Key> >
 class _LIBCPP_TEMPLATE_VIS set
@@ -423,6 +451,7 @@ public:
     typedef value_type&                              reference;
     typedef const value_type&                        const_reference;
 
+    static_assert(sizeof(__diagnose_non_const_comparator<_Key, _Compare>()), "");
     static_assert((is_same<typename allocator_type::value_type, value_type>::value),
                   "Allocator::value_type must be same type as value_type");
 
@@ -448,6 +477,11 @@ public:
     typedef __insert_return_type<iterator, node_type> insert_return_type;
 #endif
 
+    template <class _Key2, class _Compare2, class _Alloc2>
+        friend class _LIBCPP_TEMPLATE_VIS set;
+    template <class _Key2, class _Compare2, class _Alloc2>
+        friend class _LIBCPP_TEMPLATE_VIS multiset;
+
     _LIBCPP_INLINE_VISIBILITY
     set()
         _NOEXCEPT_(
@@ -486,7 +520,7 @@ public:
 
 #if _LIBCPP_STD_VER > 11
         template <class _InputIterator>
-        _LIBCPP_INLINE_VISIBILITY 
+        _LIBCPP_INLINE_VISIBILITY
         set(_InputIterator __f, _InputIterator __l, const allocator_type& __a)
             : set(__f, __l, key_compare(), __a) {}
 #endif
@@ -542,7 +576,7 @@ public:
         }
 
 #if _LIBCPP_STD_VER > 11
-    _LIBCPP_INLINE_VISIBILITY 
+    _LIBCPP_INLINE_VISIBILITY
     set(initializer_list<value_type> __il, const allocator_type& __a)
         : set(__il, key_compare(), __a) {}
 #endif
@@ -680,6 +714,38 @@ public:
     {
         return __tree_.template __node_handle_extract<node_type>(__it);
     }
+    template <class _Compare2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(set<key_type, _Compare2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_unique(__source.__tree_);
+    }
+    template <class _Compare2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(set<key_type, _Compare2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_unique(__source.__tree_);
+    }
+    template <class _Compare2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(multiset<key_type, _Compare2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_unique(__source.__tree_);
+    }
+    template <class _Compare2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(multiset<key_type, _Compare2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_unique(__source.__tree_);
+    }
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
@@ -852,6 +918,13 @@ swap(set<_Key, _Compare, _Allocator>& __x,
     __x.swap(__y);
 }
 
+#if _LIBCPP_STD_VER > 17
+template <class _Key, class _Compare, class _Allocator, class _Predicate>
+inline _LIBCPP_INLINE_VISIBILITY
+void erase_if(set<_Key, _Compare, _Allocator>& __c, _Predicate __pred)
+{ __libcpp_erase_if_container(__c, __pred); }
+#endif
+
 template <class _Key, class _Compare = less<_Key>,
           class _Allocator = allocator<_Key> >
 class _LIBCPP_TEMPLATE_VIS multiset
@@ -866,6 +939,7 @@ public:
     typedef value_type&                              reference;
     typedef const value_type&                        const_reference;
 
+    static_assert(sizeof(__diagnose_non_const_comparator<_Key, _Compare>()), "");
     static_assert((is_same<typename allocator_type::value_type, value_type>::value),
                   "Allocator::value_type must be same type as value_type");
 
@@ -890,6 +964,11 @@ public:
     typedef __set_node_handle<typename __base::__node, allocator_type> node_type;
 #endif
 
+    template <class _Key2, class _Compare2, class _Alloc2>
+        friend class _LIBCPP_TEMPLATE_VIS set;
+    template <class _Key2, class _Compare2, class _Alloc2>
+        friend class _LIBCPP_TEMPLATE_VIS multiset;
+
     // construct/copy/destroy:
     _LIBCPP_INLINE_VISIBILITY
     multiset()
@@ -920,7 +999,7 @@ public:
 
 #if _LIBCPP_STD_VER > 11
         template <class _InputIterator>
-        _LIBCPP_INLINE_VISIBILITY 
+        _LIBCPP_INLINE_VISIBILITY
         multiset(_InputIterator __f, _InputIterator __l, const allocator_type& __a)
             : multiset(__f, __l, key_compare(), __a) {}
 #endif
@@ -984,7 +1063,7 @@ public:
         }
 
 #if _LIBCPP_STD_VER > 11
-    _LIBCPP_INLINE_VISIBILITY 
+    _LIBCPP_INLINE_VISIBILITY
     multiset(initializer_list<value_type> __il, const allocator_type& __a)
         : multiset(__il, key_compare(), __a) {}
 #endif
@@ -1121,6 +1200,38 @@ public:
     {
         return __tree_.template __node_handle_extract<node_type>(__it);
     }
+    template <class _Compare2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(multiset<key_type, _Compare2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_multi(__source.__tree_);
+    }
+    template <class _Compare2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(multiset<key_type, _Compare2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_multi(__source.__tree_);
+    }
+    template <class _Compare2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(set<key_type, _Compare2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_multi(__source.__tree_);
+    }
+    template <class _Compare2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(set<key_type, _Compare2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_multi(__source.__tree_);
+    }
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
@@ -1294,6 +1405,13 @@ swap(multiset<_Key, _Compare, _Allocator>& __x,
     __x.swap(__y);
 }
 
+#if _LIBCPP_STD_VER > 17
+template <class _Key, class _Compare, class _Allocator, class _Predicate>
+inline _LIBCPP_INLINE_VISIBILITY
+void erase_if(multiset<_Key, _Compare, _Allocator>& __c, _Predicate __pred)
+{ __libcpp_erase_if_container(__c, __pred); }
+#endif
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif  // _LIBCPP_SET
diff --git a/include/shared_mutex b/include/shared_mutex
index a7735d6732c5..3daf74d26c74 100644
--- a/include/shared_mutex
+++ b/include/shared_mutex
@@ -124,6 +124,7 @@ template <class Mutex>
 */
 
 #include <__config>
+#include <version>
 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
@@ -143,7 +144,8 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-struct _LIBCPP_TYPE_VIS _LIBCPP_AVAILABILITY_SHARED_MUTEX __shared_mutex_base
+struct _LIBCPP_TYPE_VIS _LIBCPP_AVAILABILITY_SHARED_MUTEX _LIBCPP_THREAD_SAFETY_ANNOTATION(capability("shared_mutex"))
+__shared_mutex_base
 {
     mutex               __mut_;
     condition_variable  __gate1_;
@@ -160,14 +162,14 @@ struct _LIBCPP_TYPE_VIS _LIBCPP_AVAILABILITY_SHARED_MUTEX __shared_mutex_base
     __shared_mutex_base& operator=(const __shared_mutex_base&) = delete;
 
     // Exclusive ownership
-    void lock(); // blocking
-    bool try_lock();
-    void unlock();
+    void lock() _LIBCPP_THREAD_SAFETY_ANNOTATION(acquire_capability()); // blocking
+    bool try_lock() _LIBCPP_THREAD_SAFETY_ANNOTATION(try_acquire_capability(true));
+    void unlock() _LIBCPP_THREAD_SAFETY_ANNOTATION(release_capability());
 
     // Shared ownership
-    void lock_shared(); // blocking
-    bool try_lock_shared();
-    void unlock_shared();
+    void lock_shared() _LIBCPP_THREAD_SAFETY_ANNOTATION(acquire_shared_capability()); // blocking
+    bool try_lock_shared() _LIBCPP_THREAD_SAFETY_ANNOTATION(try_acquire_shared_capability(true));
+    void unlock_shared() _LIBCPP_THREAD_SAFETY_ANNOTATION(release_shared_capability());
 
 //     typedef implementation-defined native_handle_type; // See 30.2.3
 //     native_handle_type native_handle(); // See 30.2.3
diff --git a/include/span b/include/span
index db8f836f9184..cebe98760f27 100644
--- a/include/span
+++ b/include/span
@@ -23,27 +23,13 @@ inline constexpr ptrdiff_t dynamic_extent = -1;
 template <class ElementType, ptrdiff_t Extent = dynamic_extent>
     class span;
 
-// [span.comparison], span comparison operators
-template <class T, ptrdiff_t X, class U, ptrdiff_t Y>
-    constexpr bool operator==(span<T, X> l, span<U, Y> r);
-template <class T, ptrdiff_t X, class U, ptrdiff_t Y>
-    constexpr bool operator!=(span<T, X> l, span<U, Y> r);
-template <class T, ptrdiff_t X, class U, ptrdiff_t Y>
-    constexpr bool operator<(span<T, X> l, span<U, Y> r);
-template <class T, ptrdiff_t X, class U, ptrdiff_t Y>
-    constexpr bool operator<=(span<T, X> l, span<U, Y> r);
-template <class T, ptrdiff_t X, class U, ptrdiff_t Y>
-    constexpr bool operator>(span<T, X> l, span<U, Y> r);
-template <class T, ptrdiff_t X, class U, ptrdiff_t Y>
-    constexpr bool operator>=(span<T, X> l, span<U, Y> r);
-
 // [span.objectrep], views of object representation
 template <class ElementType, ptrdiff_t Extent>
-    span<const byte, ((Extent == dynamic_extent) ? dynamic_extent : 
+    span<const byte, ((Extent == dynamic_extent) ? dynamic_extent :
         (static_cast<ptrdiff_t>(sizeof(ElementType)) * Extent))> as_bytes(span<ElementType, Extent> s) noexcept;
 
 template <class ElementType, ptrdiff_t Extent>
-    span<      byte, ((Extent == dynamic_extent) ? dynamic_extent : 
+    span<      byte, ((Extent == dynamic_extent) ? dynamic_extent :
         (static_cast<ptrdiff_t>(sizeof(ElementType)) * Extent))> as_writable_bytes(span<ElementType, Extent> s) noexcept;
 
 
@@ -123,7 +109,7 @@ private:
 
 template<class T, size_t N>
     span(T (&)[N]) -> span<T, N>;
-  
+
 template<class T, size_t N>
     span(array<T, N>&) -> span<T, N>;
 
@@ -194,8 +180,8 @@ struct __is_span_compatible_container<_Tp, _ElementType,
             decltype(size(declval<_Tp>())),
         // remove_pointer_t<decltype(data(cont))>(*)[] is convertible to ElementType(*)[]
             typename enable_if<
-				is_convertible_v<remove_pointer_t<decltype(data(declval<_Tp &>()))>(*)[],
-				                 _ElementType(*)[]>,
+                is_convertible_v<remove_pointer_t<decltype(data(declval<_Tp &>()))>(*)[],
+                                 _ElementType(*)[]>,
                 nullptr_t>::type
         >>
     : public true_type {};
@@ -221,7 +207,7 @@ public:
     static constexpr index_type extent = _Extent;
     static_assert (_Extent >= 0, "Can't have a span with an extent < 0");
 
-// [span.cons], span constructors, copy, assignment, and destructor 
+// [span.cons], span constructors, copy, assignment, and destructor
     _LIBCPP_INLINE_VISIBILITY constexpr span() noexcept : __data{nullptr}
     { static_assert(_Extent == 0, "Can't default construct a statically sized span with size > 0"); }
 
@@ -242,7 +228,7 @@ public:
         constexpr span(      _Container& __c,
             enable_if_t<__is_span_compatible_container<_Container, _Tp>::value, nullptr_t> = nullptr)
         : __data{_VSTD::data(__c)}
-        { _LIBCPP_ASSERT(_Extent == _VSTD::size(__c), "size mismatch in span's constructor (container))"); }
+        { _LIBCPP_ASSERT(_Extent == _VSTD::size(__c), "size mismatch in span's constructor (container)"); }
 
     template <class _Container>
     inline _LIBCPP_INLINE_VISIBILITY
@@ -287,7 +273,7 @@ public:
         static_assert(_Count <= _Extent, "Count out of range in span::last()");
         return {data() + size() - _Count, _Count};
     }
-    
+
     _LIBCPP_INLINE_VISIBILITY
     constexpr span<element_type, dynamic_extent> first(index_type __count) const noexcept
     {
@@ -315,7 +301,7 @@ public:
     inline _LIBCPP_INLINE_VISIBILITY
     constexpr span<element_type, dynamic_extent>
        subspan(index_type __offset, index_type __count = dynamic_extent) const noexcept
-    { 
+    {
         _LIBCPP_ASSERT( __offset >= 0 && __offset <= size(), "Offset out of range in span::subspan(offset, count)");
         _LIBCPP_ASSERT((__count  >= 0 && __count  <= size()) || __count == dynamic_extent, "Count out of range in span::subspan(offset, count)");
         if (__count == dynamic_extent)
@@ -331,14 +317,14 @@ public:
     _LIBCPP_INLINE_VISIBILITY constexpr reference operator[](index_type __idx) const noexcept
     {
         _LIBCPP_ASSERT(__idx >= 0 && __idx < size(), "span<T,N>[] index out of bounds");
-		return __data[__idx];
-	}
+        return __data[__idx];
+    }
 
     _LIBCPP_INLINE_VISIBILITY constexpr reference operator()(index_type __idx) const noexcept
     {
         _LIBCPP_ASSERT(__idx >= 0 && __idx < size(), "span<T,N>() index out of bounds");
-		return __data[__idx];
-	}
+        return __data[__idx];
+    }
 
     _LIBCPP_INLINE_VISIBILITY constexpr pointer data()                         const noexcept { return __data; }
 
@@ -358,7 +344,7 @@ public:
         __data = __other.__data;
         __other.__data = __p;
     }
-    
+
     _LIBCPP_INLINE_VISIBILITY span<const byte, _Extent * sizeof(element_type)> __as_bytes() const noexcept
     { return {reinterpret_cast<const byte *>(data()), size_bytes()}; }
 
@@ -392,7 +378,7 @@ public:
 
     static constexpr index_type extent = dynamic_extent;
 
-// [span.cons], span constructors, copy, assignment, and destructor 
+// [span.cons], span constructors, copy, assignment, and destructor
     _LIBCPP_INLINE_VISIBILITY constexpr span() noexcept : __data{nullptr}, __size{0} {}
 
     constexpr span           (const span&) noexcept = default;
@@ -408,7 +394,7 @@ public:
     template <size_t _Sz>
     inline _LIBCPP_INLINE_VISIBILITY
         constexpr span(array<value_type, _Sz>& __arr)       noexcept : __data{__arr.data()}, __size{_Sz} {}
-    
+
     template <size_t _Sz>
     inline _LIBCPP_INLINE_VISIBILITY
         constexpr span(const array<value_type, _Sz>& __arr) noexcept : __data{__arr.data()}, __size{_Sz} {}
@@ -440,7 +426,7 @@ public:
     inline _LIBCPP_INLINE_VISIBILITY
         constexpr span<element_type, _Count> first() const noexcept
     {
-        static_assert(_Count >= 0);
+        static_assert(_Count >= 0, "Count must be >= 0 in span::first()");
         _LIBCPP_ASSERT(_Count <= size(), "Count out of range in span::first()");
         return {data(), _Count};
     }
@@ -449,7 +435,7 @@ public:
     inline _LIBCPP_INLINE_VISIBILITY
         constexpr span<element_type, _Count> last() const noexcept
     {
-        static_assert(_Count >= 0);
+        static_assert(_Count >= 0, "Count must be >= 0 in span::last()");
         _LIBCPP_ASSERT(_Count <= size(), "Count out of range in span::last()");
         return {data() + size() - _Count, _Count};
     }
@@ -460,7 +446,7 @@ public:
         _LIBCPP_ASSERT(__count >= 0 && __count <= size(), "Count out of range in span::first(count)");
         return {data(), __count};
     }
-        
+
     _LIBCPP_INLINE_VISIBILITY
     constexpr span<element_type, dynamic_extent> last (index_type __count) const noexcept
     {
@@ -480,7 +466,7 @@ public:
     constexpr span<element_type, dynamic_extent>
     inline _LIBCPP_INLINE_VISIBILITY
        subspan(index_type __offset, index_type __count = dynamic_extent) const noexcept
-    { 
+    {
         _LIBCPP_ASSERT( __offset >= 0 && __offset <= size(), "Offset out of range in span::subspan(offset, count)");
         _LIBCPP_ASSERT((__count  >= 0 && __count  <= size()) || __count == dynamic_extent, "count out of range in span::subspan(offset, count)");
         if (__count == dynamic_extent)
@@ -496,14 +482,14 @@ public:
     _LIBCPP_INLINE_VISIBILITY constexpr reference operator[](index_type __idx) const noexcept
     {
         _LIBCPP_ASSERT(__idx >= 0 && __idx < size(), "span<T>[] index out of bounds");
-		return __data[__idx];
-	}
+        return __data[__idx];
+    }
 
     _LIBCPP_INLINE_VISIBILITY constexpr reference operator()(index_type __idx) const noexcept
     {
         _LIBCPP_ASSERT(__idx >= 0 && __idx < size(), "span<T>() index out of bounds");
-		return __data[__idx];
-	}
+        return __data[__idx];
+    }
 
     _LIBCPP_INLINE_VISIBILITY constexpr pointer data()                         const noexcept { return __data; }
 
@@ -539,39 +525,9 @@ private:
     index_type __size;
 };
 
-template <class _Tp1, ptrdiff_t _Extent1, class _Tp2, ptrdiff_t _Extent2>
-    constexpr bool
-    operator==(const span<_Tp1, _Extent1>& __lhs, const span<_Tp2, _Extent2>& __rhs)
-    { return equal(__lhs.begin(), __lhs.end(), __rhs.begin(), __rhs.end()); }
-
-template <class _Tp1, ptrdiff_t _Extent1, class _Tp2, ptrdiff_t _Extent2>
-    constexpr bool
-    operator!=(const span<_Tp1, _Extent1>& __lhs, const span<_Tp2, _Extent2>& __rhs)
-    { return !(__rhs == __lhs); }
-
-template <class _Tp1, ptrdiff_t _Extent1, class _Tp2, ptrdiff_t _Extent2>
-    constexpr bool
-    operator< (const span<_Tp1, _Extent1>& __lhs, const span<_Tp2, _Extent2>& __rhs)
-    { return lexicographical_compare (__lhs.begin(), __lhs.end(), __rhs.begin(), __rhs.end()); }
-    
-template <class _Tp1, ptrdiff_t _Extent1, class _Tp2, ptrdiff_t _Extent2>
-    constexpr bool
-    operator<=(const span<_Tp1, _Extent1>& __lhs, const span<_Tp2, _Extent2>& __rhs)
-    { return !(__rhs < __lhs); }
-
-template <class _Tp1, ptrdiff_t _Extent1, class _Tp2, ptrdiff_t _Extent2>
-    constexpr bool
-    operator> (const span<_Tp1, _Extent1>& __lhs, const span<_Tp2, _Extent2>& __rhs)
-    { return __rhs < __lhs; }
-    
-template <class _Tp1, ptrdiff_t _Extent1, class _Tp2, ptrdiff_t _Extent2>
-    constexpr bool
-    operator>=(const span<_Tp1, _Extent1>& __lhs, const span<_Tp2, _Extent2>& __rhs)
-    { return !(__lhs < __rhs); }
-
 //  as_bytes & as_writeable_bytes
 template <class _Tp, ptrdiff_t _Extent>
-    auto as_bytes(span<_Tp, _Extent> __s) noexcept 
+    auto as_bytes(span<_Tp, _Extent> __s) noexcept
     -> decltype(__s.__as_bytes())
     { return __s.__as_bytes(); }
 
@@ -588,7 +544,7 @@ template <class _Tp, ptrdiff_t _Extent>
 //  Deduction guides
 template<class _Tp, size_t _Sz>
     span(_Tp (&)[_Sz]) -> span<_Tp, _Sz>;
-  
+
 template<class _Tp, size_t _Sz>
     span(array<_Tp, _Sz>&) -> span<_Tp, _Sz>;
 
diff --git a/include/sstream b/include/sstream
index b01f47b6872c..9c3ee13bfbad 100644
--- a/include/sstream
+++ b/include/sstream
@@ -473,12 +473,12 @@ basic_stringbuf<_CharT, _Traits, _Allocator>::str(const string_type& __s)
         {
             while (__sz > INT_MAX)
             {
-            	this->pbump(INT_MAX);
-            	__sz -= INT_MAX;
+                this->pbump(INT_MAX);
+                __sz -= INT_MAX;
             }
             if (__sz > 0)
-            	this->pbump(__sz);
-    	}
+                this->pbump(__sz);
+        }
     }
 }
 
diff --git a/include/stddef.h b/include/stddef.h
index faf8552d8ce7..f65065d869a8 100644
--- a/include/stddef.h
+++ b/include/stddef.h
@@ -54,7 +54,7 @@ using std::nullptr_t;
 
 // Re-use the compiler's <stddef.h> max_align_t where possible.
 #if !defined(__CLANG_MAX_ALIGN_T_DEFINED) && !defined(_GCC_MAX_ALIGN_T) && \
-    !defined(__DEFINED_max_align_t)
+    !defined(__DEFINED_max_align_t) && !defined(__NetBSD__)
 typedef long double max_align_t;
 #endif
 
diff --git a/include/stdexcept b/include/stdexcept
index 6533497e4ea5..3ec79349aa92 100644
--- a/include/stdexcept
+++ b/include/stdexcept
@@ -192,7 +192,7 @@ void __throw_logic_error(const char*__msg)
     throw logic_error(__msg);
 #else
     ((void)__msg);
-	  _VSTD::abort();
+    _VSTD::abort();
 #endif
 }
 
@@ -203,7 +203,7 @@ void __throw_domain_error(const char*__msg)
     throw domain_error(__msg);
 #else
     ((void)__msg);
-	  _VSTD::abort();
+    _VSTD::abort();
 #endif
 }
 
@@ -214,7 +214,7 @@ void __throw_invalid_argument(const char*__msg)
     throw invalid_argument(__msg);
 #else
     ((void)__msg);
-	  _VSTD::abort();
+    _VSTD::abort();
 #endif
 }
 
@@ -225,7 +225,7 @@ void __throw_length_error(const char*__msg)
     throw length_error(__msg);
 #else
     ((void)__msg);
-	  _VSTD::abort();
+    _VSTD::abort();
 #endif
 }
 
@@ -236,7 +236,7 @@ void __throw_out_of_range(const char*__msg)
     throw out_of_range(__msg);
 #else
     ((void)__msg);
-	  _VSTD::abort();
+    _VSTD::abort();
 #endif
 }
 
@@ -247,7 +247,7 @@ void __throw_range_error(const char*__msg)
     throw range_error(__msg);
 #else
     ((void)__msg);
-	  _VSTD::abort();
+    _VSTD::abort();
 #endif
 }
 
diff --git a/include/streambuf b/include/streambuf
index 37a653237161..dd293dc639b9 100644
--- a/include/streambuf
+++ b/include/streambuf
@@ -138,7 +138,7 @@ public:
     virtual ~basic_streambuf();
 
     // 27.6.2.2.1 locales:
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     locale pubimbue(const locale& __loc) {
         imbue(__loc);
         locale __r = __loc_;
@@ -146,70 +146,70 @@ public:
         return __r;
     }
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     locale getloc() const { return __loc_; }
 
     // 27.6.2.2.2 buffer and positioning:
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     basic_streambuf* pubsetbuf(char_type* __s, streamsize __n)
     { return setbuf(__s, __n); }
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     pos_type pubseekoff(off_type __off, ios_base::seekdir __way,
                         ios_base::openmode __which = ios_base::in | ios_base::out)
     { return seekoff(__off, __way, __which); }
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     pos_type pubseekpos(pos_type __sp,
                         ios_base::openmode __which = ios_base::in | ios_base::out)
     { return seekpos(__sp, __which); }
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     int pubsync() { return sync(); }
 
     // Get and put areas:
     // 27.6.2.2.3 Get area:
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     streamsize in_avail() {
         if (__ninp_ < __einp_)
             return static_cast<streamsize>(__einp_ - __ninp_);
         return showmanyc();
     }
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     int_type snextc() {
         if (sbumpc() == traits_type::eof())
             return traits_type::eof();
         return sgetc();
     }
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     int_type sbumpc() {
         if (__ninp_ == __einp_)
             return uflow();
         return traits_type::to_int_type(*__ninp_++);
     }
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     int_type sgetc() {
         if (__ninp_ == __einp_)
             return underflow();
         return traits_type::to_int_type(*__ninp_);
     }
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     streamsize sgetn(char_type* __s, streamsize __n)
     { return xsgetn(__s, __n); }
 
     // 27.6.2.2.4 Putback:
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     int_type sputbackc(char_type __c) {
         if (__binp_ == __ninp_ || !traits_type::eq(__c, __ninp_[-1]))
             return pbackfail(traits_type::to_int_type(__c));
         return traits_type::to_int_type(*--__ninp_);
     }
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     int_type sungetc() {
         if (__binp_ == __ninp_)
           return pbackfail();
@@ -217,7 +217,7 @@ public:
     }
 
     // 27.6.2.2.5 Put area:
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     int_type sputc(char_type __c) {
         if (__nout_ == __eout_)
             return overflow(traits_type::to_int_type(__c));
@@ -225,7 +225,7 @@ public:
         return traits_type::to_int_type(__c);
     }
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     streamsize sputn(const char_type* __s, streamsize __n)
     { return xsputn(__s, __n); }
 
@@ -240,10 +240,10 @@ protected:
     _LIBCPP_INLINE_VISIBILITY char_type* gptr()  const {return __ninp_;}
     _LIBCPP_INLINE_VISIBILITY char_type* egptr() const {return __einp_;}
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     void gbump(int __n) { __ninp_ += __n; }
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     void setg(char_type* __gbeg, char_type* __gnext, char_type* __gend) {
         __binp_ = __gbeg;
         __ninp_ = __gnext;
@@ -255,13 +255,13 @@ protected:
     _LIBCPP_INLINE_VISIBILITY char_type* pptr()  const {return __nout_;}
     _LIBCPP_INLINE_VISIBILITY char_type* epptr() const {return __eout_;}
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     void pbump(int __n) { __nout_ += __n; }
 
     _LIBCPP_INLINE_VISIBILITY
     void __pbump(streamsize __n) { __nout_ += __n; }
 
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     void setp(char_type* __pbeg, char_type* __pend) {
         __bout_ = __nout_ = __pbeg;
         __eout_ = __pend;
@@ -486,7 +486,7 @@ basic_streambuf<_CharT, _Traits>::overflow(int_type)
     return traits_type::eof();
 }
 
-#ifndef _LIBCPP_AVAILABILITY_NO_STREAMS_EXTERN_TEMPLATE
+#ifndef _LIBCPP_DO_NOT_ASSUME_STREAMS_EXPLICIT_INSTANTIATION_IN_DYLIB
 _LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS basic_streambuf<char>)
 _LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS basic_streambuf<wchar_t>)
 
diff --git a/include/string b/include/string
index 162e54058c42..fb838d1e5dd2 100644
--- a/include/string
+++ b/include/string
@@ -437,6 +437,11 @@ template<class charT, class traits, class Allocator>
 basic_istream<charT, traits>&
 getline(basic_istream<charT, traits>& is, basic_string<charT, traits, Allocator>& str);
 
+template<class charT, class traits, class Allocator, class U>
+void erase(basic_string<charT, traits, Allocator>& c, const U& value); // C++20
+template<class charT, class traits, class Allocator, class Predicate>
+void erase_if(basic_string<charT, traits, Allocator>& c, Predicate pred); // C++20
+
 typedef basic_string<char>    string;
 typedef basic_string<wchar_t> wstring;
 typedef basic_string<char16_t> u16string;
@@ -510,6 +515,7 @@ basic_string<char32_t> operator "" s( const char32_t *str, size_t len ); // C++1
 #include <type_traits>
 #include <initializer_list>
 #include <__functional_base>
+#include <version>
 #ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
 #include <cstdint>
 #endif
@@ -579,6 +585,7 @@ basic_string<_CharT, _Traits, _Allocator>
 operator+(_CharT __x, const basic_string<_CharT,_Traits,_Allocator>& __y);
 
 template<class _CharT, class _Traits, class _Allocator>
+inline _LIBCPP_INLINE_VISIBILITY
 basic_string<_CharT, _Traits, _Allocator>
 operator+(const basic_string<_CharT, _Traits, _Allocator>& __x, const _CharT* __y);
 
@@ -806,7 +813,9 @@ public:
     basic_string(basic_string&& __str, const allocator_type& __a);
 #endif  // _LIBCPP_CXX03_LANG
 
+#ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
     template <class = typename enable_if<__is_allocator<_Allocator>::value, nullptr_t>::type>
+#endif
     _LIBCPP_INLINE_VISIBILITY
     basic_string(const _CharT* __s) {
       _LIBCPP_ASSERT(__s != nullptr, "basic_string(const char*) detected nullptr");
@@ -816,7 +825,9 @@ public:
 #   endif
     }
 
+#ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
     template <class = typename enable_if<__is_allocator<_Allocator>::value, nullptr_t>::type>
+#endif
         _LIBCPP_INLINE_VISIBILITY
         basic_string(const _CharT* __s, const _Allocator& __a);
 
@@ -827,7 +838,9 @@ public:
     _LIBCPP_INLINE_VISIBILITY
     basic_string(size_type __n, _CharT __c);
 
+#ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
     template <class = typename enable_if<__is_allocator<_Allocator>::value, nullptr_t>::type>
+#endif
         _LIBCPP_INLINE_VISIBILITY
         basic_string(size_type __n, _CharT __c, const _Allocator& __a);
 
@@ -948,7 +961,11 @@ public:
     void resize(size_type __n, value_type __c);
     _LIBCPP_INLINE_VISIBILITY void resize(size_type __n) {resize(__n, value_type());}
 
-    void reserve(size_type __res_arg = 0);
+    void reserve(size_type __res_arg);
+    _LIBCPP_INLINE_VISIBILITY void __resize_default_init(size_type __n);
+
+    _LIBCPP_INLINE_VISIBILITY
+    void reserve() _NOEXCEPT {reserve(0);}
     _LIBCPP_INLINE_VISIBILITY
     void shrink_to_fit() _NOEXCEPT {reserve();}
     _LIBCPP_INLINE_VISIBILITY
@@ -1002,6 +1019,10 @@ public:
     basic_string& append(const value_type* __s, size_type __n);
     basic_string& append(const value_type* __s);
     basic_string& append(size_type __n, value_type __c);
+
+    _LIBCPP_INLINE_VISIBILITY
+    void __append_default_init(size_type __n);
+
     template <class _ForwardIterator>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
     basic_string& __append_forward_unsafe(_ForwardIterator, _ForwardIterator);
@@ -1664,7 +1685,7 @@ basic_string(basic_string_view<_CharT, _Traits>, _Sz, _Sz, const _Allocator& = _
 
                   
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 void
 basic_string<_CharT, _Traits, _Allocator>::__invalidate_all_iterators()
 {
@@ -1674,7 +1695,7 @@ basic_string<_CharT, _Traits, _Allocator>::__invalidate_all_iterators()
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 void
 basic_string<_CharT, _Traits, _Allocator>::__invalidate_iterators_past(size_type
 #if _LIBCPP_DEBUG_LEVEL >= 2
@@ -1704,7 +1725,7 @@ basic_string<_CharT, _Traits, _Allocator>::__invalidate_iterators_past(size_type
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_string<_CharT, _Traits, _Allocator>::basic_string()
     _NOEXCEPT_(is_nothrow_default_constructible<allocator_type>::value)
 {
@@ -1715,7 +1736,7 @@ basic_string<_CharT, _Traits, _Allocator>::basic_string()
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_string<_CharT, _Traits, _Allocator>::basic_string(const allocator_type& __a)
 #if _LIBCPP_STD_VER <= 14
         _NOEXCEPT_(is_nothrow_copy_constructible<allocator_type>::value)
@@ -1780,7 +1801,9 @@ basic_string<_CharT, _Traits, _Allocator>::__init(const value_type* __s, size_ty
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+#ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
 template <class>
+#endif
 basic_string<_CharT, _Traits, _Allocator>::basic_string(const _CharT* __s, const _Allocator& __a)
     : __r_(__second_tag(), __a)
 {
@@ -1792,7 +1815,7 @@ basic_string<_CharT, _Traits, _Allocator>::basic_string(const _CharT* __s, const
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_string<_CharT, _Traits, _Allocator>::basic_string(const _CharT* __s, size_type __n)
 {
     _LIBCPP_ASSERT(__n == 0 || __s != nullptr, "basic_string(const char*, n) detected nullptr");
@@ -1803,7 +1826,7 @@ basic_string<_CharT, _Traits, _Allocator>::basic_string(const _CharT* __s, size_
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_string<_CharT, _Traits, _Allocator>::basic_string(const _CharT* __s, size_type __n, const _Allocator& __a)
     : __r_(__second_tag(), __a)
 {
@@ -1844,7 +1867,7 @@ basic_string<_CharT, _Traits, _Allocator>::basic_string(
 #ifndef _LIBCPP_CXX03_LANG
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_string<_CharT, _Traits, _Allocator>::basic_string(basic_string&& __str)
 #if _LIBCPP_STD_VER <= 14
         _NOEXCEPT_(is_nothrow_move_constructible<allocator_type>::value)
@@ -1862,7 +1885,7 @@ basic_string<_CharT, _Traits, _Allocator>::basic_string(basic_string&& __str)
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_string<_CharT, _Traits, _Allocator>::basic_string(basic_string&& __str, const allocator_type& __a)
     : __r_(__second_tag(), __a)
 {
@@ -1907,7 +1930,7 @@ basic_string<_CharT, _Traits, _Allocator>::__init(size_type __n, value_type __c)
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_string<_CharT, _Traits, _Allocator>::basic_string(size_type __n, _CharT __c)
 {
     __init(__n, __c);
@@ -1917,7 +1940,9 @@ basic_string<_CharT, _Traits, _Allocator>::basic_string(size_type __n, _CharT __
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+#ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
 template <class>
+#endif
 basic_string<_CharT, _Traits, _Allocator>::basic_string(size_type __n, _CharT __c, const _Allocator& __a)
     : __r_(__second_tag(), __a)
 {
@@ -1943,7 +1968,7 @@ basic_string<_CharT, _Traits, _Allocator>::basic_string(const basic_string& __st
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_string<_CharT, _Traits, _Allocator>::basic_string(const basic_string& __str, size_type __pos,
                                                         const _Allocator& __a)
     : __r_(__second_tag(), __a)
@@ -2054,7 +2079,7 @@ basic_string<_CharT, _Traits, _Allocator>::__init(_ForwardIterator __first, _For
 
 template <class _CharT, class _Traits, class _Allocator>
 template<class _InputIterator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_string<_CharT, _Traits, _Allocator>::basic_string(_InputIterator __first, _InputIterator __last)
 {
     __init(__first, __last);
@@ -2065,7 +2090,7 @@ basic_string<_CharT, _Traits, _Allocator>::basic_string(_InputIterator __first,
 
 template <class _CharT, class _Traits, class _Allocator>
 template<class _InputIterator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_string<_CharT, _Traits, _Allocator>::basic_string(_InputIterator __first, _InputIterator __last,
                                                         const allocator_type& __a)
     : __r_(__second_tag(), __a)
@@ -2079,7 +2104,7 @@ basic_string<_CharT, _Traits, _Allocator>::basic_string(_InputIterator __first,
 #ifndef _LIBCPP_CXX03_LANG
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_string<_CharT, _Traits, _Allocator>::basic_string(
     initializer_list<_CharT> __il)
 {
@@ -2090,7 +2115,7 @@ basic_string<_CharT, _Traits, _Allocator>::basic_string(
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 
 basic_string<_CharT, _Traits, _Allocator>::basic_string(
     initializer_list<_CharT> __il, const _Allocator& __a)
@@ -2254,7 +2279,7 @@ basic_string<_CharT, _Traits, _Allocator>::operator=(const basic_string& __str)
 #ifndef _LIBCPP_CXX03_LANG
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 void
 basic_string<_CharT, _Traits, _Allocator>::__move_assign(basic_string& __str, false_type)
     _NOEXCEPT_(__alloc_traits::is_always_equal::value)
@@ -2266,7 +2291,7 @@ basic_string<_CharT, _Traits, _Allocator>::__move_assign(basic_string& __str, fa
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 void
 basic_string<_CharT, _Traits, _Allocator>::__move_assign(basic_string& __str, true_type)
 #if _LIBCPP_STD_VER > 14
@@ -2282,7 +2307,7 @@ basic_string<_CharT, _Traits, _Allocator>::__move_assign(basic_string& __str, tr
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::operator=(basic_string&& __str)
     _NOEXCEPT_((__noexcept_move_assign_container<_Allocator, __alloc_traits>::value))
@@ -2416,6 +2441,23 @@ basic_string<_CharT, _Traits, _Allocator>::append(size_type __n, value_type __c)
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+inline void
+basic_string<_CharT, _Traits, _Allocator>::__append_default_init(size_type __n)
+{
+    if (__n)
+    {
+        size_type __cap = capacity();
+        size_type __sz = size();
+        if (__cap - __sz < __n)
+            __grow_by(__cap, __sz + __n - __cap, __sz, __sz, 0);
+        pointer __p = __get_pointer();
+        __sz += __n;
+        __set_size(__sz);
+        traits_type::assign(__p[__sz], value_type());
+    }
+}
+
+template <class _CharT, class _Traits, class _Allocator>
 void
 basic_string<_CharT, _Traits, _Allocator>::push_back(value_type __c)
 {
@@ -2499,7 +2541,7 @@ basic_string<_CharT, _Traits, _Allocator>::__append_forward_unsafe(
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::append(const basic_string& __str)
 {
@@ -2676,7 +2718,7 @@ basic_string<_CharT, _Traits, _Allocator>::insert(const_iterator __pos, _Forward
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::insert(size_type __pos1, const basic_string& __str)
 {
@@ -2746,7 +2788,7 @@ basic_string<_CharT, _Traits, _Allocator>::insert(const_iterator __pos, value_ty
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::iterator
 basic_string<_CharT, _Traits, _Allocator>::insert(const_iterator __pos, size_type __n, value_type __c)
 {
@@ -2866,7 +2908,7 @@ basic_string<_CharT, _Traits, _Allocator>::replace(const_iterator __i1, const_it
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::replace(size_type __pos1, size_type __n1, const basic_string& __str)
 {
@@ -2910,7 +2952,7 @@ basic_string<_CharT, _Traits, _Allocator>::replace(size_type __pos, size_type __
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::replace(const_iterator __i1, const_iterator __i2, const basic_string& __str)
 {
@@ -2919,7 +2961,7 @@ basic_string<_CharT, _Traits, _Allocator>::replace(const_iterator __i1, const_it
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::replace(const_iterator __i1, const_iterator __i2, const value_type* __s, size_type __n)
 {
@@ -2927,7 +2969,7 @@ basic_string<_CharT, _Traits, _Allocator>::replace(const_iterator __i1, const_it
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::replace(const_iterator __i1, const_iterator __i2, const value_type* __s)
 {
@@ -2935,7 +2977,7 @@ basic_string<_CharT, _Traits, _Allocator>::replace(const_iterator __i1, const_it
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::replace(const_iterator __i1, const_iterator __i2, size_type __n, value_type __c)
 {
@@ -2967,7 +3009,7 @@ basic_string<_CharT, _Traits, _Allocator>::erase(size_type __pos, size_type __n)
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::iterator
 basic_string<_CharT, _Traits, _Allocator>::erase(const_iterator __pos)
 {
@@ -2985,7 +3027,7 @@ basic_string<_CharT, _Traits, _Allocator>::erase(const_iterator __pos)
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::iterator
 basic_string<_CharT, _Traits, _Allocator>::erase(const_iterator __first, const_iterator __last)
 {
@@ -3002,7 +3044,7 @@ basic_string<_CharT, _Traits, _Allocator>::erase(const_iterator __first, const_i
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 void
 basic_string<_CharT, _Traits, _Allocator>::pop_back()
 {
@@ -3024,7 +3066,7 @@ basic_string<_CharT, _Traits, _Allocator>::pop_back()
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 void
 basic_string<_CharT, _Traits, _Allocator>::clear() _NOEXCEPT
 {
@@ -3042,7 +3084,7 @@ basic_string<_CharT, _Traits, _Allocator>::clear() _NOEXCEPT
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 void
 basic_string<_CharT, _Traits, _Allocator>::__erase_to_end(size_type __pos)
 {
@@ -3071,7 +3113,18 @@ basic_string<_CharT, _Traits, _Allocator>::resize(size_type __n, value_type __c)
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline void
+basic_string<_CharT, _Traits, _Allocator>::__resize_default_init(size_type __n)
+{
+    size_type __sz = size();
+    if (__n > __sz) {
+       __append_default_init(__n - __sz);
+    } else
+        __erase_to_end(__n);
+}
+
+template <class _CharT, class _Traits, class _Allocator>
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::max_size() const _NOEXCEPT
 {
@@ -3147,7 +3200,7 @@ basic_string<_CharT, _Traits, _Allocator>::reserve(size_type __res_arg)
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::const_reference
 basic_string<_CharT, _Traits, _Allocator>::operator[](size_type __pos) const _NOEXCEPT
 {
@@ -3156,7 +3209,7 @@ basic_string<_CharT, _Traits, _Allocator>::operator[](size_type __pos) const _NO
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::reference
 basic_string<_CharT, _Traits, _Allocator>::operator[](size_type __pos) _NOEXCEPT
 {
@@ -3183,7 +3236,7 @@ basic_string<_CharT, _Traits, _Allocator>::at(size_type __n)
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::reference
 basic_string<_CharT, _Traits, _Allocator>::front()
 {
@@ -3192,7 +3245,7 @@ basic_string<_CharT, _Traits, _Allocator>::front()
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::const_reference
 basic_string<_CharT, _Traits, _Allocator>::front() const
 {
@@ -3201,7 +3254,7 @@ basic_string<_CharT, _Traits, _Allocator>::front() const
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::reference
 basic_string<_CharT, _Traits, _Allocator>::back()
 {
@@ -3210,7 +3263,7 @@ basic_string<_CharT, _Traits, _Allocator>::back()
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::const_reference
 basic_string<_CharT, _Traits, _Allocator>::back() const
 {
@@ -3231,7 +3284,7 @@ basic_string<_CharT, _Traits, _Allocator>::copy(value_type* __s, size_type __n,
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 basic_string<_CharT, _Traits, _Allocator>
 basic_string<_CharT, _Traits, _Allocator>::substr(size_type __pos, size_type __n) const
 {
@@ -3239,7 +3292,7 @@ basic_string<_CharT, _Traits, _Allocator>::substr(size_type __pos, size_type __n
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 void
 basic_string<_CharT, _Traits, _Allocator>::swap(basic_string& __str)
 #if _LIBCPP_STD_VER >= 14
@@ -3287,7 +3340,7 @@ basic_string<_CharT, _Traits, _Allocator>::find(const value_type* __s,
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find(const basic_string& __str,
                                                 size_type __pos) const _NOEXCEPT
@@ -3312,7 +3365,7 @@ basic_string<_CharT, _Traits, _Allocator>::find(const _Tp &__t,
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find(const value_type* __s,
                                                 size_type __pos) const _NOEXCEPT
@@ -3345,7 +3398,7 @@ basic_string<_CharT, _Traits, _Allocator>::rfind(const value_type* __s,
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::rfind(const basic_string& __str,
                                                  size_type __pos) const _NOEXCEPT
@@ -3370,7 +3423,7 @@ basic_string<_CharT, _Traits, _Allocator>::rfind(const _Tp& __t,
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::rfind(const value_type* __s,
                                                  size_type __pos) const _NOEXCEPT
@@ -3403,7 +3456,7 @@ basic_string<_CharT, _Traits, _Allocator>::find_first_of(const value_type* __s,
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_first_of(const basic_string& __str,
                                                          size_type __pos) const _NOEXCEPT
@@ -3428,7 +3481,7 @@ basic_string<_CharT, _Traits, _Allocator>::find_first_of(const _Tp& __t,
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_first_of(const value_type* __s,
                                                          size_type __pos) const _NOEXCEPT
@@ -3439,7 +3492,7 @@ basic_string<_CharT, _Traits, _Allocator>::find_first_of(const value_type* __s,
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_first_of(value_type __c,
                                                          size_type __pos) const _NOEXCEPT
@@ -3461,7 +3514,7 @@ basic_string<_CharT, _Traits, _Allocator>::find_last_of(const value_type* __s,
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_last_of(const basic_string& __str,
                                                         size_type __pos) const _NOEXCEPT
@@ -3486,7 +3539,7 @@ basic_string<_CharT, _Traits, _Allocator>::find_last_of(const _Tp& __t,
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_last_of(const value_type* __s,
                                                         size_type __pos) const _NOEXCEPT
@@ -3497,7 +3550,7 @@ basic_string<_CharT, _Traits, _Allocator>::find_last_of(const value_type* __s,
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_last_of(value_type __c,
                                                         size_type __pos) const _NOEXCEPT
@@ -3519,7 +3572,7 @@ basic_string<_CharT, _Traits, _Allocator>::find_first_not_of(const value_type* _
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_first_not_of(const basic_string& __str,
                                                              size_type __pos) const _NOEXCEPT
@@ -3544,7 +3597,7 @@ basic_string<_CharT, _Traits, _Allocator>::find_first_not_of(const _Tp& __t,
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_first_not_of(const value_type* __s,
                                                              size_type __pos) const _NOEXCEPT
@@ -3555,7 +3608,7 @@ basic_string<_CharT, _Traits, _Allocator>::find_first_not_of(const value_type* _
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_first_not_of(value_type __c,
                                                              size_type __pos) const _NOEXCEPT
@@ -3578,7 +3631,7 @@ basic_string<_CharT, _Traits, _Allocator>::find_last_not_of(const value_type* __
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_last_not_of(const basic_string& __str,
                                                             size_type __pos) const _NOEXCEPT
@@ -3603,7 +3656,7 @@ basic_string<_CharT, _Traits, _Allocator>::find_last_not_of(const _Tp& __t,
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_last_not_of(const value_type* __s,
                                                             size_type __pos) const _NOEXCEPT
@@ -3614,7 +3667,7 @@ basic_string<_CharT, _Traits, _Allocator>::find_last_not_of(const value_type* __
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_last_not_of(value_type __c,
                                                             size_type __pos) const _NOEXCEPT
@@ -3649,7 +3702,7 @@ basic_string<_CharT, _Traits, _Allocator>::compare(const _Tp& __t) const
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 int
 basic_string<_CharT, _Traits, _Allocator>::compare(const basic_string& __str) const _NOEXCEPT
 {
@@ -3695,7 +3748,7 @@ basic_string<_CharT, _Traits, _Allocator>::compare(size_type __pos1,
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 int
 basic_string<_CharT, _Traits, _Allocator>::compare(size_type __pos1,
                                                    size_type __n1,
@@ -3753,7 +3806,7 @@ basic_string<_CharT, _Traits, _Allocator>::compare(size_type __pos1,
 // __invariants
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 bool
 basic_string<_CharT, _Traits, _Allocator>::__invariants() const
 {
@@ -3771,7 +3824,7 @@ basic_string<_CharT, _Traits, _Allocator>::__invariants() const
 // __clear_and_shrink
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline
 void 
 basic_string<_CharT, _Traits, _Allocator>::__clear_and_shrink() _NOEXCEPT
 {
@@ -4025,6 +4078,7 @@ operator+(_CharT __lhs, const basic_string<_CharT,_Traits,_Allocator>& __rhs)
 }
 
 template<class _CharT, class _Traits, class _Allocator>
+inline
 basic_string<_CharT, _Traits, _Allocator>
 operator+(const basic_string<_CharT, _Traits, _Allocator>& __lhs, const _CharT* __rhs)
 {
@@ -4121,11 +4175,13 @@ swap(basic_string<_CharT, _Traits, _Allocator>& __lhs,
     __lhs.swap(__rhs);
 }
 
-#ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
+#ifndef _LIBCPP_NO_HAS_CHAR8_T
+typedef basic_string<char8_t> u8string;
+#endif
 
+#ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
 typedef basic_string<char16_t> u16string;
 typedef basic_string<char32_t> u32string;
-
 #endif  // _LIBCPP_HAS_NO_UNICODE_CHARS
 
 _LIBCPP_FUNC_VIS int                stoi  (const string& __str, size_t* __idx = 0, int __base = 10);
@@ -4225,6 +4281,18 @@ getline(basic_istream<_CharT, _Traits>&& __is,
 
 #endif  // _LIBCPP_CXX03_LANG
 
+#if _LIBCPP_STD_VER > 17
+template<class _CharT, class _Traits, class _Allocator, class _Up>
+inline _LIBCPP_INLINE_VISIBILITY
+void erase(basic_string<_CharT, _Traits, _Allocator>& __str, const _Up& __v)
+{ __str.erase(_VSTD::remove(__str.begin(), __str.end(), __v), __str.end()); }
+
+template<class _CharT, class _Traits, class _Allocator, class _Predicate>
+inline _LIBCPP_INLINE_VISIBILITY
+void erase_if(basic_string<_CharT, _Traits, _Allocator>& __str, _Predicate __pred)
+{ __str.erase(_VSTD::remove_if(__str.begin(), __str.end(), __pred), __str.end()); }
+#endif
+
 #if _LIBCPP_DEBUG_LEVEL >= 2
 
 template<class _CharT, class _Traits, class _Allocator>
@@ -4282,6 +4350,14 @@ inline namespace literals
         return basic_string<wchar_t> (__str, __len);
     }
 
+#ifndef _LIBCPP_NO_HAS_CHAR8_T
+    inline _LIBCPP_INLINE_VISIBILITY
+    basic_string<char8_t> operator "" s(const char8_t *__str, size_t __len) _NOEXCEPT
+    {
+        return basic_string<char8_t> (__str, __len);
+    }
+#endif
+
     inline _LIBCPP_INLINE_VISIBILITY
     basic_string<char16_t> operator "" s( const char16_t *__str, size_t __len )
     {
diff --git a/include/string_view b/include/string_view
index 6377aeb6d648..7d783122f123 100644
--- a/include/string_view
+++ b/include/string_view
@@ -178,6 +178,7 @@ namespace std {
 #include <iterator>
 #include <limits>
 #include <stdexcept>
+#include <version>
 #include <__debug>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -768,6 +769,9 @@ bool operator>=(typename common_type<basic_string_view<_CharT, _Traits> >::type
 }
 
 typedef basic_string_view<char>     string_view;
+#ifndef _LIBCPP_NO_HAS_CHAR8_T
+typedef basic_string_view<char8_t>  u8string_view;
+#endif
 typedef basic_string_view<char16_t> u16string_view;
 typedef basic_string_view<char32_t> u32string_view;
 typedef basic_string_view<wchar_t>  wstring_view;
@@ -777,17 +781,12 @@ template<class _CharT, class _Traits>
 struct _LIBCPP_TEMPLATE_VIS hash<basic_string_view<_CharT, _Traits> >
     : public unary_function<basic_string_view<_CharT, _Traits>, size_t>
 {
-    size_t operator()(const basic_string_view<_CharT, _Traits> __val) const _NOEXCEPT;
+    _LIBCPP_INLINE_VISIBILITY
+    size_t operator()(const basic_string_view<_CharT, _Traits> __val) const _NOEXCEPT {
+        return __do_string_hash(__val.data(), __val.data() + __val.size());
+    }
 };
 
-template<class _CharT, class _Traits>
-size_t
-hash<basic_string_view<_CharT, _Traits> >::operator()(
-        const basic_string_view<_CharT, _Traits> __val) const _NOEXCEPT
-{
-    return __do_string_hash(__val.data(), __val.data() + __val.size());
-}
-
 
 #if _LIBCPP_STD_VER > 11 
 inline namespace literals
@@ -806,6 +805,14 @@ inline namespace literals
         return basic_string_view<wchar_t> (__str, __len);
     }
 
+#ifndef _LIBCPP_NO_HAS_CHAR8_T
+    inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
+    basic_string_view<char8_t> operator "" sv(const char8_t *__str, size_t __len) _NOEXCEPT
+    {
+        return basic_string_view<char8_t> (__str, __len);
+    }
+#endif
+
     inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
     basic_string_view<char16_t> operator "" sv(const char16_t *__str, size_t __len) _NOEXCEPT
     {
diff --git a/include/support/win32/locale_win32.h b/include/support/win32/locale_win32.h
index 68682c9624b8..c7c6d786cb83 100644
--- a/include/support/win32/locale_win32.h
+++ b/include/support/win32/locale_win32.h
@@ -35,8 +35,8 @@ public:
         : __locale(nullptr), __locale_str(nullptr) {}
     locale_t(std::nullptr_t)
         : __locale(nullptr), __locale_str(nullptr) {}
-    locale_t(_locale_t __locale, const char* __locale_str)
-        : __locale(__locale), __locale_str(__locale_str) {}
+    locale_t(_locale_t __xlocale, const char* __xlocale_str)
+        : __locale(__xlocale), __locale_str(__xlocale_str) {}
 
     friend bool operator==(const locale_t& __left, const locale_t& __right) {
         return __left.__locale == __right.__locale;
diff --git a/include/thread b/include/thread
index 0629d70efda4..8c0115f8708d 100644
--- a/include/thread
+++ b/include/thread
@@ -151,7 +151,7 @@ class __thread_specific_ptr
     __thread_specific_ptr(const __thread_specific_ptr&);
     __thread_specific_ptr& operator=(const __thread_specific_ptr&);
 
-    static void _LIBCPP_TLS_DESTRUCTOR_CC __at_thread_exit(void*);
+    _LIBCPP_HIDDEN static void _LIBCPP_TLS_DESTRUCTOR_CC __at_thread_exit(void*);
 
 public:
     typedef _Tp* pointer;
diff --git a/include/tuple b/include/tuple
index b3a17e7b7354..4cc69030b9ab 100644
--- a/include/tuple
+++ b/include/tuple
@@ -65,7 +65,7 @@ public:
     template <class U1, class U2>
         tuple& operator=(const pair<U1, U2>&); // iff sizeof...(T) == 2
     template <class U1, class U2>
-        tuple& operator=(pair<U1, U2>&&); //iffsizeof...(T) == 2
+        tuple& operator=(pair<U1, U2>&&); // iff sizeof...(T) == 2
 
     void swap(tuple&) noexcept(AND(swap(declval<T&>(), declval<T&>())...));
 };
@@ -84,8 +84,8 @@ template <class T, class Tuple>
   constexpr T make_from_tuple(Tuple&& t); // C++17
 
 // 20.4.1.4, tuple helper classes:
-template <class T> class tuple_size; // undefined
-template <class... T> class tuple_size<tuple<T...>>;
+template <class T> struct tuple_size; // undefined
+template <class... T> struct tuple_size<tuple<T...>>;
 template <class T>
  inline constexpr size_t tuple_size_v = tuple_size<T>::value; // C++17
 template <size_t I, class T> class tuple_element; // undefined
@@ -141,6 +141,7 @@ template <class... Types>
 #include <type_traits>
 #include <__functional_base>
 #include <utility>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
@@ -1077,30 +1078,12 @@ namespace {
   _LIBCPP_INLINE_VAR constexpr __ignore_t<unsigned char> ignore = __ignore_t<unsigned char>();
 }
 
-template <class _Tp>
-struct __make_tuple_return_impl
-{
-    typedef _Tp type;
-};
-
-template <class _Tp>
-struct __make_tuple_return_impl<reference_wrapper<_Tp> >
-{
-    typedef _Tp& type;
-};
-
-template <class _Tp>
-struct __make_tuple_return
-{
-    typedef typename __make_tuple_return_impl<typename decay<_Tp>::type>::type type;
-};
-
 template <class... _Tp>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
-tuple<typename __make_tuple_return<_Tp>::type...>
+tuple<typename __unwrap_ref_decay<_Tp>::type...>
 make_tuple(_Tp&&... __t)
 {
-    return tuple<typename __make_tuple_return<_Tp>::type...>(_VSTD::forward<_Tp>(__t)...);
+    return tuple<typename __unwrap_ref_decay<_Tp>::type...>(_VSTD::forward<_Tp>(__t)...);
 }
 
 template <class... _Tp>
diff --git a/include/type_traits b/include/type_traits
index 30bfb161c0d4..8b30511a4ecb 100644
--- a/include/type_traits
+++ b/include/type_traits
@@ -75,6 +75,10 @@ namespace std
     template <class T> struct remove_pointer;
     template <class T> struct add_pointer;
 
+    template<class T> struct type_identity;                     // C++20
+    template<class T>
+      using type_identity_t = typename type_identity<T>::type;  // C++20
+
     // Integral properties:
     template <class T> struct is_signed;
     template <class T> struct is_unsigned;
@@ -400,6 +404,7 @@ namespace std
 */
 #include <__config>
 #include <cstddef>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
@@ -704,6 +709,9 @@ template <>          struct __libcpp_is_integral<char>               : public tr
 template <>          struct __libcpp_is_integral<signed char>        : public true_type {};
 template <>          struct __libcpp_is_integral<unsigned char>      : public true_type {};
 template <>          struct __libcpp_is_integral<wchar_t>            : public true_type {};
+#ifndef _LIBCPP_NO_HAS_CHAR8_T
+template <>          struct __libcpp_is_integral<char8_t>            : public true_type {};
+#endif
 #ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
 template <>          struct __libcpp_is_integral<char16_t>           : public true_type {};
 template <>          struct __libcpp_is_integral<char32_t>           : public true_type {};
@@ -733,12 +741,6 @@ _LIBCPP_INLINE_VAR _LIBCPP_CONSTEXPR bool is_integral_v
 // is_floating_point
 
 template <class _Tp> struct __libcpp_is_floating_point              : public false_type {};
-#ifdef __clang__
-template <>          struct __libcpp_is_floating_point<__fp16>      : public true_type {};
-#endif
-#ifdef __FLT16_MANT_DIG__
-template <>          struct __libcpp_is_floating_point<_Float16>    : public true_type {};
-#endif
 template <>          struct __libcpp_is_floating_point<float>       : public true_type {};
 template <>          struct __libcpp_is_floating_point<double>      : public true_type {};
 template <>          struct __libcpp_is_floating_point<long double> : public true_type {};
@@ -1226,6 +1228,12 @@ template <class _Tp> struct _LIBCPP_TEMPLATE_VIS add_pointer
 template <class _Tp> using add_pointer_t = typename add_pointer<_Tp>::type;
 #endif
 
+// type_identity
+#if _LIBCPP_STD_VER > 17
+template<class _Tp> struct type_identity { typedef _Tp type; };
+template<class _Tp> using type_identity_t = typename type_identity<_Tp>::type;
+#endif
+
 // is_signed
 
 template <class _Tp, bool = is_integral<_Tp>::value>
@@ -1644,7 +1652,7 @@ _LIBCPP_INLINE_VAR _LIBCPP_CONSTEXPR bool has_unique_object_representations_v
 // alignment_of
 
 template <class _Tp> struct _LIBCPP_TEMPLATE_VIS alignment_of
-    : public integral_constant<size_t, __alignof__(_Tp)> {};
+    : public integral_constant<size_t, _LIBCPP_ALIGNOF(_Tp)> {};
 
 #if _LIBCPP_STD_VER > 14 && !defined(_LIBCPP_HAS_NO_VARIABLE_TEMPLATES)
 template <class _Tp>
@@ -1674,7 +1682,7 @@ struct __nat
 template <class _Tp>
 struct __align_type
 {
-    static const size_t value = alignment_of<_Tp>::value;
+    static const size_t value = _LIBCPP_PREFERRED_ALIGNOF(_Tp);
     typedef _Tp type;
 };
 
@@ -1807,8 +1815,8 @@ struct __static_max<_I0, _I1, _In...>
 template <size_t _Len, class _Type0, class ..._Types>
 struct aligned_union
 {
-    static const size_t alignment_value = __static_max<__alignof__(_Type0),
-                                                       __alignof__(_Types)...>::value;
+    static const size_t alignment_value = __static_max<_LIBCPP_PREFERRED_ALIGNOF(_Type0),
+                                                       _LIBCPP_PREFERRED_ALIGNOF(_Types)...>::value;
     static const size_t __len = __static_max<_Len, sizeof(_Type0),
                                              sizeof(_Types)...>::value;
     typedef typename aligned_storage<__len, alignment_value>::type type;
@@ -4748,7 +4756,6 @@ struct __has_operator_addressof
 
 #if _LIBCPP_STD_VER > 14
 
-#define __cpp_lib_void_t 201411
 template <class...> using void_t = void;
 
 # ifndef _LIBCPP_HAS_NO_VARIADICS
diff --git a/include/typeinfo b/include/typeinfo
index f32ea6e76f17..8411532860bd 100644
--- a/include/typeinfo
+++ b/include/typeinfo
@@ -73,12 +73,8 @@ public:
 #include <vcruntime_typeinfo.h>
 #else
 
-#if !defined(_LIBCPP_ABI_MICROSOFT)
-#if defined(_LIBCPP_NONUNIQUE_RTTI_BIT)
-#define _LIBCPP_HAS_NONUNIQUE_TYPEINFO
-#else
-#define _LIBCPP_HAS_UNIQUE_TYPEINFO
-#endif
+#if defined(_LIBCPP_NONUNIQUE_RTTI_BIT) && !defined(_LIBCPP_ABI_MICROSOFT)
+#   define _LIBCPP_HAS_NONUNIQUE_TYPEINFO
 #endif
 
 namespace std  // purposefully not using versioning namespace
@@ -232,7 +228,7 @@ void __throw_bad_cast()
 #ifndef _LIBCPP_NO_EXCEPTIONS
     throw bad_cast();
 #else
-	_VSTD::abort();
+    _VSTD::abort();
 #endif
 }
 _LIBCPP_END_NAMESPACE_STD
diff --git a/include/unordered_map b/include/unordered_map
index 348f57923b52..6035b05dc61c 100644
--- a/include/unordered_map
+++ b/include/unordered_map
@@ -153,6 +153,15 @@ public:
     iterator erase(const_iterator first, const_iterator last);
     void clear() noexcept;
 
+    template<class H2, class P2>
+      void merge(unordered_map<Key, T, H2, P2, Allocator>& source);         // C++17
+    template<class H2, class P2>
+      void merge(unordered_map<Key, T, H2, P2, Allocator>&& source);        // C++17
+    template<class H2, class P2>
+      void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source);    // C++17
+    template<class H2, class P2>
+      void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source);   // C++17
+
     void swap(unordered_map&)
         noexcept(
             (!allocator_type::propagate_on_container_swap::value ||
@@ -325,6 +334,15 @@ public:
     iterator erase(const_iterator first, const_iterator last);
     void clear() noexcept;
 
+    template<class H2, class P2>
+      void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source);    // C++17
+    template<class H2, class P2>
+      void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source);   // C++17
+    template<class H2, class P2>
+      void merge(unordered_map<Key, T, H2, P2, Allocator>& source);         // C++17
+    template<class H2, class P2>
+      void merge(unordered_map<Key, T, H2, P2, Allocator>&& source);        // C++17
+
     void swap(unordered_multimap&)
         noexcept(
             (!allocator_type::propagate_on_container_swap::value ||
@@ -366,6 +384,12 @@ template <class Key, class T, class Hash, class Pred, class Alloc>
               unordered_multimap<Key, T, Hash, Pred, Alloc>& y)
               noexcept(noexcept(x.swap(y)));
 
+template <class K, class T, class H, class P, class A, class Predicate>
+    void erase_if(unordered_set<K, T, H, P, A>& c, Predicate pred);       // C++20
+
+template <class K, class T, class H, class P, class A, class Predicate>
+    void erase_if(unordered_multiset<K, T, H, P, A>& c, Predicate pred);  // C++20
+
 template <class Key, class T, class Hash, class Pred, class Alloc>
     bool
     operator==(const unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
@@ -386,6 +410,7 @@ template <class Key, class T, class Hash, class Pred, class Alloc>
 #include <functional>
 #include <stdexcept>
 #include <tuple>
+#include <version>
 
 #include <__debug>
 
@@ -395,7 +420,8 @@ template <class Key, class T, class Hash, class Pred, class Alloc>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <class _Key, class _Cp, class _Hash, bool _IsEmpty>
+template <class _Key, class _Cp, class _Hash,
+          bool = is_empty<_Hash>::value && !__libcpp_is_final<_Hash>::value>
 class __unordered_map_hasher
     : private _Hash
 {
@@ -463,7 +489,8 @@ swap(__unordered_map_hasher<_Key, _Cp, _Hash, __b>& __x,
     __x.swap(__y);
 }
 
-template <class _Key, class _Cp, class _Pred, bool _IsEmpty>
+template <class _Key, class _Cp, class _Pred,
+          bool = is_empty<_Pred>::value && !__libcpp_is_final<_Pred>::value>
 class __unordered_map_equal
     : private _Pred
 {
@@ -807,6 +834,9 @@ public:
     template <class> friend class _LIBCPP_TEMPLATE_VIS __hash_const_local_iterator;
 };
 
+template <class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>
+class unordered_multimap;
+
 template <class _Key, class _Tp, class _Hash = hash<_Key>, class _Pred = equal_to<_Key>,
           class _Alloc = allocator<pair<const _Key, _Tp> > >
 class _LIBCPP_TEMPLATE_VIS unordered_map
@@ -823,6 +853,7 @@ public:
     typedef const value_type&                              const_reference;
     static_assert((is_same<value_type, typename allocator_type::value_type>::value),
                   "Invalid allocator::value_type");
+    static_assert(sizeof(__diagnose_unordered_container_requirements<_Key, _Hash, _Pred>(0)), "");
 
 private:
     typedef __hash_value_type<key_type, mapped_type>                 __value_type;
@@ -864,6 +895,11 @@ public:
     typedef __insert_return_type<iterator, node_type> insert_return_type;
 #endif
 
+    template <class _Key2, class _Tp2, class _Hash2, class _Pred2, class _Alloc2>
+        friend class _LIBCPP_TEMPLATE_VIS unordered_map;
+    template <class _Key2, class _Tp2, class _Hash2, class _Pred2, class _Alloc2>
+        friend class _LIBCPP_TEMPLATE_VIS unordered_multimap;
+
     _LIBCPP_INLINE_VISIBILITY
     unordered_map()
         _NOEXCEPT_(is_nothrow_default_constructible<__table>::value)
@@ -1187,6 +1223,39 @@ public:
         return __table_.template __node_handle_extract<node_type>(
             __it.__i_);
     }
+
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_map<key_type, mapped_type, _H2, _P2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_unique(__source.__table_);
+    }
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_map<key_type, mapped_type, _H2, _P2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_unique(__source.__table_);
+    }
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_multimap<key_type, mapped_type, _H2, _P2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_unique(__source.__table_);
+    }
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_multimap<key_type, mapped_type, _H2, _P2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_unique(__source.__table_);
+    }
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
@@ -1563,6 +1632,13 @@ swap(unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>& __x,
     __x.swap(__y);
 }
 
+#if _LIBCPP_STD_VER > 17
+template <class _Key, class _Tp, class _Hash, class _Pred, class _Alloc, class _Predicate>
+inline _LIBCPP_INLINE_VISIBILITY
+void erase_if(unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>& __c, _Predicate __pred)
+{ __libcpp_erase_if_container(__c, __pred); }
+#endif
+
 template <class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>
 bool
 operator==(const unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>& __x,
@@ -1607,6 +1683,7 @@ public:
     typedef const value_type&                              const_reference;
     static_assert((is_same<value_type, typename allocator_type::value_type>::value),
                   "Invalid allocator::value_type");
+    static_assert(sizeof(__diagnose_unordered_container_requirements<_Key, _Hash, _Pred>(0)), "");
 
 private:
     typedef __hash_value_type<key_type, mapped_type>                 __value_type;
@@ -1645,6 +1722,11 @@ public:
     typedef __map_node_handle<__node, allocator_type> node_type;
 #endif
 
+    template <class _Key2, class _Tp2, class _Hash2, class _Pred2, class _Alloc2>
+        friend class _LIBCPP_TEMPLATE_VIS unordered_map;
+    template <class _Key2, class _Tp2, class _Hash2, class _Pred2, class _Alloc2>
+        friend class _LIBCPP_TEMPLATE_VIS unordered_multimap;
+
     _LIBCPP_INLINE_VISIBILITY
     unordered_multimap()
         _NOEXCEPT_(is_nothrow_default_constructible<__table>::value)
@@ -1846,6 +1928,39 @@ public:
         return __table_.template __node_handle_extract<node_type>(
             __it.__i_);
     }
+
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_multimap<key_type, mapped_type, _H2, _P2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_multi(__source.__table_);
+    }
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_multimap<key_type, mapped_type, _H2, _P2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_multi(__source.__table_);
+    }
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_map<key_type, mapped_type, _H2, _P2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_multi(__source.__table_);
+    }
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_map<key_type, mapped_type, _H2, _P2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_multi(__source.__table_);
+    }
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
@@ -2141,6 +2256,13 @@ swap(unordered_multimap<_Key, _Tp, _Hash, _Pred, _Alloc>& __x,
     __x.swap(__y);
 }
 
+#if _LIBCPP_STD_VER > 17
+template <class _Key, class _Tp, class _Hash, class _Pred, class _Alloc, class _Predicate>
+inline _LIBCPP_INLINE_VISIBILITY
+void erase_if(unordered_multimap<_Key, _Tp, _Hash, _Pred, _Alloc>& __c, _Predicate __pred)
+{ __libcpp_erase_if_container(__c, __pred); }
+#endif
+
 template <class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>
 bool
 operator==(const unordered_multimap<_Key, _Tp, _Hash, _Pred, _Alloc>& __x,
diff --git a/include/unordered_set b/include/unordered_set
index 9b8560da494a..b4e61da89eec 100644
--- a/include/unordered_set
+++ b/include/unordered_set
@@ -127,6 +127,15 @@ public:
     iterator erase(const_iterator first, const_iterator last);
     void clear() noexcept;
 
+    template<class H2, class P2>
+      void merge(unordered_set<Key, H2, P2, Allocator>& source);         // C++17
+    template<class H2, class P2>
+      void merge(unordered_set<Key, H2, P2, Allocator>&& source);        // C++17
+    template<class H2, class P2>
+      void merge(unordered_multiset<Key, H2, P2, Allocator>& source);    // C++17
+    template<class H2, class P2>
+      void merge(unordered_multiset<Key, H2, P2, Allocator>&& source);   // C++17
+
     void swap(unordered_set&)
        noexcept(allocator_traits<Allocator>::is_always_equal::value &&
                  noexcept(swap(declval<hasher&>(), declval<hasher&>())) &&
@@ -282,6 +291,15 @@ public:
     iterator erase(const_iterator first, const_iterator last);
     void clear() noexcept;
 
+    template<class H2, class P2>
+      void merge(unordered_multiset<Key, H2, P2, Allocator>& source);    // C++17
+    template<class H2, class P2>
+      void merge(unordered_multiset<Key, H2, P2, Allocator>&& source);   // C++17
+    template<class H2, class P2>
+      void merge(unordered_set<Key, H2, P2, Allocator>& source);         // C++17
+    template<class H2, class P2>
+      void merge(unordered_set<Key, H2, P2, Allocator>&& source);        // C++17
+
     void swap(unordered_multiset&)
        noexcept(allocator_traits<Allocator>::is_always_equal::value &&
                  noexcept(swap(declval<hasher&>(), declval<hasher&>())) &&
@@ -321,6 +339,13 @@ template <class Value, class Hash, class Pred, class Alloc>
               unordered_multiset<Value, Hash, Pred, Alloc>& y)
               noexcept(noexcept(x.swap(y)));
 
+template <class K, class T, class H, class P, class A, class Predicate>
+    void erase_if(unordered_set<K, T, H, P, A>& c, Predicate pred);       // C++20
+
+template <class K, class T, class H, class P, class A, class Predicate>
+    void erase_if(unordered_multiset<K, T, H, P, A>& c, Predicate pred);  // C++20
+
+
 template <class Value, class Hash, class Pred, class Alloc>
     bool
     operator==(const unordered_multiset<Value, Hash, Pred, Alloc>& x,
@@ -338,6 +363,7 @@ template <class Value, class Hash, class Pred, class Alloc>
 #include <__hash_table>
 #include <__node_handle>
 #include <functional>
+#include <version>
 
 #include <__debug>
 
@@ -347,6 +373,9 @@ template <class Value, class Hash, class Pred, class Alloc>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+template <class _Value, class _Hash, class _Pred, class _Alloc>
+class unordered_multiset;
+
 template <class _Value, class _Hash = hash<_Value>, class _Pred = equal_to<_Value>,
           class _Alloc = allocator<_Value> >
 class _LIBCPP_TEMPLATE_VIS unordered_set
@@ -362,6 +391,7 @@ public:
     typedef const value_type&                                          const_reference;
     static_assert((is_same<value_type, typename allocator_type::value_type>::value),
                   "Invalid allocator::value_type");
+    static_assert(sizeof(__diagnose_unordered_container_requirements<_Value, _Hash, _Pred>(0)), "");
 
 private:
     typedef __hash_table<value_type, hasher, key_equal, allocator_type> __table;
@@ -384,6 +414,11 @@ public:
     typedef __insert_return_type<iterator, node_type> insert_return_type;
 #endif
 
+    template <class _Value2, class _Hash2, class _Pred2, class _Alloc2>
+        friend class _LIBCPP_TEMPLATE_VIS unordered_set;
+    template <class _Value2, class _Hash2, class _Pred2, class _Alloc2>
+        friend class _LIBCPP_TEMPLATE_VIS unordered_multiset;
+
     _LIBCPP_INLINE_VISIBILITY
     unordered_set()
         _NOEXCEPT_(is_nothrow_default_constructible<__table>::value)
@@ -589,6 +624,39 @@ public:
     {
         return __table_.template __node_handle_extract<node_type>(__it);
     }
+
+    template<class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_set<key_type, _H2, _P2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __table_.__node_handle_merge_unique(__source.__table_);
+    }
+    template<class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_set<key_type, _H2, _P2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __table_.__node_handle_merge_unique(__source.__table_);
+    }
+    template<class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_multiset<key_type, _H2, _P2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __table_.__node_handle_merge_unique(__source.__table_);
+    }
+    template<class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_multiset<key_type, _H2, _P2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __table_.__node_handle_merge_unique(__source.__table_);
+    }
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
@@ -873,6 +941,13 @@ swap(unordered_set<_Value, _Hash, _Pred, _Alloc>& __x,
     __x.swap(__y);
 }
 
+#if _LIBCPP_STD_VER > 17
+template <class _Value, class _Hash, class _Pred, class _Alloc, class _Predicate>
+inline _LIBCPP_INLINE_VISIBILITY
+void erase_if(unordered_set<_Value, _Hash, _Pred, _Alloc>& __c, _Predicate __pred)
+{ __libcpp_erase_if_container(__c, __pred); }
+#endif
+
 template <class _Value, class _Hash, class _Pred, class _Alloc>
 bool
 operator==(const unordered_set<_Value, _Hash, _Pred, _Alloc>& __x,
@@ -916,6 +991,7 @@ public:
     typedef const value_type&                                          const_reference;
     static_assert((is_same<value_type, typename allocator_type::value_type>::value),
                   "Invalid allocator::value_type");
+    static_assert(sizeof(__diagnose_unordered_container_requirements<_Value, _Hash, _Pred>(0)), "");
 
 private:
     typedef __hash_table<value_type, hasher, key_equal, allocator_type> __table;
@@ -937,6 +1013,11 @@ public:
     typedef __set_node_handle<typename __table::__node, allocator_type> node_type;
 #endif
 
+    template <class _Value2, class _Hash2, class _Pred2, class _Alloc2>
+        friend class _LIBCPP_TEMPLATE_VIS unordered_set;
+    template <class _Value2, class _Hash2, class _Pred2, class _Alloc2>
+        friend class _LIBCPP_TEMPLATE_VIS unordered_multiset;
+
     _LIBCPP_INLINE_VISIBILITY
     unordered_multiset()
         _NOEXCEPT_(is_nothrow_default_constructible<__table>::value)
@@ -1101,6 +1182,39 @@ public:
     {
         return __table_.template __node_handle_extract<node_type>(__key);
     }
+
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_multiset<key_type, _H2, _P2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_multi(__source.__table_);
+    }
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_multiset<key_type, _H2, _P2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_multi(__source.__table_);
+    }
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_set<key_type, _H2, _P2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_multi(__source.__table_);
+    }
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_set<key_type, _H2, _P2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_multi(__source.__table_);
+    }
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
@@ -1397,6 +1511,13 @@ swap(unordered_multiset<_Value, _Hash, _Pred, _Alloc>& __x,
     __x.swap(__y);
 }
 
+#if _LIBCPP_STD_VER > 17
+template <class _Value, class _Hash, class _Pred, class _Alloc, class _Predicate>
+inline _LIBCPP_INLINE_VISIBILITY
+void erase_if(unordered_multiset<_Value, _Hash, _Pred, _Alloc>& __c, _Predicate __pred)
+{ __libcpp_erase_if_container(__c, __pred); }
+#endif
+
 template <class _Value, class _Hash, class _Pred, class _Alloc>
 bool
 operator==(const unordered_multiset<_Value, _Hash, _Pred, _Alloc>& __x,
diff --git a/include/utility b/include/utility
index ed9bf030d499..74bbc5cf34f9 100644
--- a/include/utility
+++ b/include/utility
@@ -103,7 +103,7 @@ swap(pair<T1, T2>& x, pair<T1, T2>& y) noexcept(noexcept(x.swap(y)));
 struct piecewise_construct_t { };
 inline constexpr piecewise_construct_t piecewise_construct = piecewise_construct_t();
 
-template <class T> class tuple_size;
+template <class T> struct tuple_size;
 template <size_t I, class T> class tuple_element;
 
 template <class T1, class T2> struct tuple_size<pair<T1, T2> >;
@@ -203,6 +203,7 @@ template <size_t I>
 #include <cstddef>
 #include <cstring>
 #include <cstdint>
+#include <version>
 #include <__debug>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -296,12 +297,13 @@ template <class _Tp>                        void as_const(const _Tp&&) = delete;
 
 struct _LIBCPP_TEMPLATE_VIS piecewise_construct_t { };
 #if defined(_LIBCPP_CXX03_LANG) || defined(_LIBCPP_BUILDING_LIBRARY)
-extern const piecewise_construct_t piecewise_construct;// = piecewise_construct_t();
+extern _LIBCPP_EXPORTED_FROM_ABI const piecewise_construct_t piecewise_construct;// = piecewise_construct_t();
 #else
 /* _LIBCPP_INLINE_VAR */ constexpr piecewise_construct_t piecewise_construct = piecewise_construct_t();
 #endif
 
 #if defined(_LIBCPP_DEPRECATED_ABI_DISABLE_PAIR_TRIVIAL_COPY_CTOR)
+template <class, class>
 struct __non_trivially_copyable_base {
   _LIBCPP_CONSTEXPR _LIBCPP_INLINE_VISIBILITY
   __non_trivially_copyable_base() _NOEXCEPT {}
@@ -313,7 +315,7 @@ struct __non_trivially_copyable_base {
 template <class _T1, class _T2>
 struct _LIBCPP_TEMPLATE_VIS pair
 #if defined(_LIBCPP_DEPRECATED_ABI_DISABLE_PAIR_TRIVIAL_COPY_CTOR)
-: private __non_trivially_copyable_base
+: private __non_trivially_copyable_base<_T1, _T2>
 #endif
 {
     typedef _T1 first_type;
@@ -408,13 +410,17 @@ struct _LIBCPP_TEMPLATE_VIS pair
             _CheckArgsDep<_Dummy>::template __enable_default<_T1, _T2>()
     > = false>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
-    pair() : first(), second() {}
+    pair() _NOEXCEPT_(is_nothrow_default_constructible<first_type>::value &&
+                      is_nothrow_default_constructible<second_type>::value)
+        : first(), second() {}
 
     template <bool _Dummy = true, _EnableB<
              _CheckArgsDep<_Dummy>::template __enable_explicit<_T1 const&, _T2 const&>()
     > = false>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     explicit pair(_T1 const& __t1, _T2 const& __t2)
+        _NOEXCEPT_(is_nothrow_copy_constructible<first_type>::value &&
+                   is_nothrow_copy_constructible<second_type>::value)
         : first(__t1), second(__t2) {}
 
     template<bool _Dummy = true, _EnableB<
@@ -422,6 +428,8 @@ struct _LIBCPP_TEMPLATE_VIS pair
     > = false>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     pair(_T1 const& __t1, _T2 const& __t2)
+        _NOEXCEPT_(is_nothrow_copy_constructible<first_type>::value &&
+                   is_nothrow_copy_constructible<second_type>::value)
         : first(__t1), second(__t2) {}
 
     template<class _U1, class _U2, _EnableB<
@@ -429,6 +437,8 @@ struct _LIBCPP_TEMPLATE_VIS pair
     > = false>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     explicit pair(_U1&& __u1, _U2&& __u2)
+        _NOEXCEPT_((is_nothrow_constructible<first_type, _U1>::value &&
+                    is_nothrow_constructible<second_type, _U2>::value))
         : first(_VSTD::forward<_U1>(__u1)), second(_VSTD::forward<_U2>(__u2)) {}
 
     template<class _U1, class _U2, _EnableB<
@@ -436,6 +446,8 @@ struct _LIBCPP_TEMPLATE_VIS pair
     > = false>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     pair(_U1&& __u1, _U2&& __u2)
+        _NOEXCEPT_((is_nothrow_constructible<first_type, _U1>::value &&
+                    is_nothrow_constructible<second_type, _U2>::value))
         : first(_VSTD::forward<_U1>(__u1)), second(_VSTD::forward<_U2>(__u2)) {}
 
     template<class _U1, class _U2, _EnableB<
@@ -443,6 +455,8 @@ struct _LIBCPP_TEMPLATE_VIS pair
     > = false>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     explicit pair(pair<_U1, _U2> const& __p)
+        _NOEXCEPT_((is_nothrow_constructible<first_type, _U1 const&>::value &&
+                    is_nothrow_constructible<second_type, _U2 const&>::value))
         : first(__p.first), second(__p.second) {}
 
     template<class _U1, class _U2, _EnableB<
@@ -450,6 +464,8 @@ struct _LIBCPP_TEMPLATE_VIS pair
     > = false>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     pair(pair<_U1, _U2> const& __p)
+        _NOEXCEPT_((is_nothrow_constructible<first_type, _U1 const&>::value &&
+                    is_nothrow_constructible<second_type, _U2 const&>::value))
         : first(__p.first), second(__p.second) {}
 
     template<class _U1, class _U2, _EnableB<
@@ -457,6 +473,8 @@ struct _LIBCPP_TEMPLATE_VIS pair
     > = false>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     explicit pair(pair<_U1, _U2>&&__p)
+        _NOEXCEPT_((is_nothrow_constructible<first_type, _U1&&>::value &&
+                    is_nothrow_constructible<second_type, _U2&&>::value))
         : first(_VSTD::forward<_U1>(__p.first)), second(_VSTD::forward<_U2>(__p.second)) {}
 
     template<class _U1, class _U2, _EnableB<
@@ -464,6 +482,8 @@ struct _LIBCPP_TEMPLATE_VIS pair
     > = false>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     pair(pair<_U1, _U2>&& __p)
+        _NOEXCEPT_((is_nothrow_constructible<first_type, _U1&&>::value &&
+                    is_nothrow_constructible<second_type, _U2&&>::value))
         : first(_VSTD::forward<_U1>(__p.first)), second(_VSTD::forward<_U2>(__p.second)) {}
 
     template<class _Tuple, _EnableB<
@@ -486,6 +506,8 @@ struct _LIBCPP_TEMPLATE_VIS pair
     _LIBCPP_INLINE_VISIBILITY
     pair(piecewise_construct_t __pc,
          tuple<_Args1...> __first_args, tuple<_Args2...> __second_args)
+        _NOEXCEPT_((is_nothrow_constructible<first_type, _Args1...>::value &&
+                    is_nothrow_constructible<second_type, _Args2...>::value))
         : pair(__pc, __first_args, __second_args,
                 typename __make_tuple_indices<sizeof...(_Args1)>::type(),
                 typename __make_tuple_indices<sizeof...(_Args2) >::type()) {}
@@ -615,32 +637,37 @@ swap(pair<_T1, _T2>& __x, pair<_T1, _T2>& __y)
     __x.swap(__y);
 }
 
-#ifndef _LIBCPP_CXX03_LANG
+template <class _Tp>
+struct __unwrap_reference { typedef _Tp type; };
 
 template <class _Tp>
-struct __make_pair_return_impl
-{
-    typedef _Tp type;
-};
+struct __unwrap_reference<reference_wrapper<_Tp> > { typedef _Tp& type; };
 
+#if _LIBCPP_STD_VER > 17
 template <class _Tp>
-struct __make_pair_return_impl<reference_wrapper<_Tp>>
-{
-    typedef _Tp& type;
-};
+struct unwrap_reference : __unwrap_reference<_Tp> { };
 
 template <class _Tp>
-struct __make_pair_return
-{
-    typedef typename __make_pair_return_impl<typename decay<_Tp>::type>::type type;
-};
+struct unwrap_ref_decay : unwrap_reference<typename decay<_Tp>::type> { };
+#endif // > C++17
+
+template <class _Tp>
+struct __unwrap_ref_decay
+#if _LIBCPP_STD_VER > 17
+    : unwrap_ref_decay<_Tp>
+#else
+    : __unwrap_reference<typename decay<_Tp>::type>
+#endif
+{ };
+
+#ifndef _LIBCPP_CXX03_LANG
 
 template <class _T1, class _T2>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
-pair<typename __make_pair_return<_T1>::type, typename __make_pair_return<_T2>::type>
+pair<typename __unwrap_ref_decay<_T1>::type, typename __unwrap_ref_decay<_T2>::type>
 make_pair(_T1&& __t1, _T2&& __t2)
 {
-    return pair<typename __make_pair_return<_T1>::type, typename __make_pair_return<_T2>::type>
+    return pair<typename __unwrap_ref_decay<_T1>::type, typename __unwrap_ref_decay<_T2>::type>
                (_VSTD::forward<_T1>(__t1), _VSTD::forward<_T2>(__t2));
 }
 
@@ -657,7 +684,7 @@ make_pair(_T1 __x, _T2 __y)
 #endif  // _LIBCPP_CXX03_LANG
 
 template <class _T1, class _T2>
-  class _LIBCPP_TEMPLATE_VIS tuple_size<pair<_T1, _T2> >
+  struct _LIBCPP_TEMPLATE_VIS tuple_size<pair<_T1, _T2> >
     : public integral_constant<size_t, 2> {};
 
 template <size_t _Ip, class _T1, class _T2>
@@ -988,8 +1015,10 @@ __murmur2_or_cityhash<_Size, 32>::operator()(const void* __key, _Size __len)
     {
     case 3:
         __h ^= __data[2] << 16;
+        _LIBCPP_FALLTHROUGH();
     case 2:
         __h ^= __data[1] << 8;
+        _LIBCPP_FALLTHROUGH();
     case 1:
         __h ^= __data[0];
         __h *= __m;
@@ -1453,7 +1482,7 @@ struct _LIBCPP_TEMPLATE_VIS hash<float>
     size_t operator()(float __v) const _NOEXCEPT
     {
         // -0.0 and 0.0 should return same hash
-       if (__v == 0)
+       if (__v == 0.0)
            return 0;
         return __scalar_hash<float>::operator()(__v);
     }
@@ -1467,7 +1496,7 @@ struct _LIBCPP_TEMPLATE_VIS hash<double>
     size_t operator()(double __v) const _NOEXCEPT
     {
         // -0.0 and 0.0 should return same hash
-       if (__v == 0)
+       if (__v == 0.0)
            return 0;
         return __scalar_hash<double>::operator()(__v);
     }
@@ -1481,7 +1510,7 @@ struct _LIBCPP_TEMPLATE_VIS hash<long double>
     size_t operator()(long double __v) const _NOEXCEPT
     {
         // -0.0 and 0.0 should return same hash
-        if (__v == 0)
+        if (__v == 0.0)
             return 0;
 #if defined(__i386__)
         // Zero out padding bits
diff --git a/include/valarray b/include/valarray
index 8d3892ad35d9..07f38c811509 100644
--- a/include/valarray
+++ b/include/valarray
@@ -803,7 +803,7 @@ public:
     // construct/destroy:
     _LIBCPP_INLINE_VISIBILITY
     valarray() : __begin_(0), __end_(0) {}
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     explicit valarray(size_t __n);
     _LIBCPP_INLINE_VISIBILITY
     valarray(const value_type& __x, size_t __n);
@@ -818,7 +818,7 @@ public:
     valarray(const gslice_array<value_type>& __ga);
     valarray(const mask_array<value_type>& __ma);
     valarray(const indirect_array<value_type>& __ia);
-    inline _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
+    inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     ~valarray();
 
     // assignment:
@@ -1054,7 +1054,8 @@ private:
     const _Up*
     end(const valarray<_Up>& __v);
 
-    void __clear();
+    _LIBCPP_INLINE_VISIBILITY
+    void __clear(size_t __capacity);
     valarray& __assign_range(const value_type* __f, const value_type* __l);
 };
 
@@ -2739,7 +2740,7 @@ __val_expr<_ValExpr>::operator valarray<__val_expr::result_type>() const
         __r.__begin_ =
             __r.__end_ =
                 static_cast<result_type*>(
-                    _VSTD::__libcpp_allocate(__n * sizeof(result_type), __alignof(result_type)));
+                    _VSTD::__libcpp_allocate(__n * sizeof(result_type), _LIBCPP_ALIGNOF(result_type)));
         for (size_t __i = 0; __i != __n; ++__r.__end_, ++__i)
             ::new (__r.__end_) result_type(__expr_[__i]);
     }
@@ -2757,18 +2758,18 @@ valarray<_Tp>::valarray(size_t __n)
     if (__n)
     {
         __begin_ = __end_ = static_cast<value_type*>(
-            _VSTD::__libcpp_allocate(__n * sizeof(value_type), __alignof(value_type)));
+            _VSTD::__libcpp_allocate(__n * sizeof(value_type), _LIBCPP_ALIGNOF(value_type)));
 #ifndef _LIBCPP_NO_EXCEPTIONS
         try
         {
 #endif  // _LIBCPP_NO_EXCEPTIONS
-            for (; __n; --__n, ++__end_)
+            for (size_t __n_left = __n; __n_left; --__n_left, ++__end_)
                 ::new (__end_) value_type();
 #ifndef _LIBCPP_NO_EXCEPTIONS
         }
         catch (...)
         {
-            __clear();
+            __clear(__n);
             throw;
         }
 #endif  // _LIBCPP_NO_EXCEPTIONS
@@ -2792,18 +2793,18 @@ valarray<_Tp>::valarray(const value_type* __p, size_t __n)
     if (__n)
     {
         __begin_ = __end_ = static_cast<value_type*>(
-            _VSTD::__libcpp_allocate(__n * sizeof(value_type), __alignof(value_type)));
+            _VSTD::__libcpp_allocate(__n * sizeof(value_type), _LIBCPP_ALIGNOF(value_type)));
 #ifndef _LIBCPP_NO_EXCEPTIONS
         try
         {
 #endif  // _LIBCPP_NO_EXCEPTIONS
-            for (; __n; ++__end_, ++__p, --__n)
+            for (size_t __n_left = __n; __n_left; ++__end_, ++__p, --__n_left)
                 ::new (__end_) value_type(*__p);
 #ifndef _LIBCPP_NO_EXCEPTIONS
         }
         catch (...)
         {
-            __clear();
+            __clear(__n);
             throw;
         }
 #endif  // _LIBCPP_NO_EXCEPTIONS
@@ -2818,7 +2819,7 @@ valarray<_Tp>::valarray(const valarray& __v)
     if (__v.size())
     {
         __begin_ = __end_ = static_cast<value_type*>(
-            _VSTD::__libcpp_allocate(__v.size() * sizeof(value_type), __alignof(value_type)));
+            _VSTD::__libcpp_allocate(__v.size() * sizeof(value_type), _LIBCPP_ALIGNOF(value_type)));
 #ifndef _LIBCPP_NO_EXCEPTIONS
         try
         {
@@ -2829,7 +2830,7 @@ valarray<_Tp>::valarray(const valarray& __v)
         }
         catch (...)
         {
-            __clear();
+            __clear(__v.size());
             throw;
         }
 #endif  // _LIBCPP_NO_EXCEPTIONS
@@ -2852,22 +2853,23 @@ valarray<_Tp>::valarray(initializer_list<value_type> __il)
     : __begin_(0),
       __end_(0)
 {
-    size_t __n = __il.size();
+    const size_t __n = __il.size();
     if (__n)
     {
         __begin_ = __end_ = static_cast<value_type*>(
-_VSTD::__libcpp_allocate(__n * sizeof(value_type), __alignof(value_type)));
+_VSTD::__libcpp_allocate(__n * sizeof(value_type), _LIBCPP_ALIGNOF(value_type)));
 #ifndef _LIBCPP_NO_EXCEPTIONS
         try
         {
 #endif  // _LIBCPP_NO_EXCEPTIONS
-            for (const value_type* __p = __il.begin(); __n; ++__end_, ++__p, --__n)
+            size_t __n_left = __n;
+            for (const value_type* __p = __il.begin(); __n_left; ++__end_, ++__p, --__n_left)
                 ::new (__end_) value_type(*__p);
 #ifndef _LIBCPP_NO_EXCEPTIONS
         }
         catch (...)
         {
-            __clear();
+            __clear(__n);
             throw;
         }
 #endif  // _LIBCPP_NO_EXCEPTIONS
@@ -2881,22 +2883,23 @@ valarray<_Tp>::valarray(const slice_array<value_type>& __sa)
     : __begin_(0),
       __end_(0)
 {
-    size_t __n = __sa.__size_;
+    const size_t __n = __sa.__size_;
     if (__n)
     {
         __begin_ = __end_ = static_cast<value_type*>(
-          _VSTD::__libcpp_allocate(__n * sizeof(value_type), __alignof(value_type)));
+          _VSTD::__libcpp_allocate(__n * sizeof(value_type), _LIBCPP_ALIGNOF(value_type)));
 #ifndef _LIBCPP_NO_EXCEPTIONS
         try
         {
 #endif  // _LIBCPP_NO_EXCEPTIONS
-            for (const value_type* __p = __sa.__vp_; __n; ++__end_, __p += __sa.__stride_, --__n)
+            size_t __n_left = __n;
+            for (const value_type* __p = __sa.__vp_; __n_left; ++__end_, __p += __sa.__stride_, --__n_left)
                 ::new (__end_) value_type(*__p);
 #ifndef _LIBCPP_NO_EXCEPTIONS
         }
         catch (...)
         {
-            __clear();
+            __clear(__n);
             throw;
         }
 #endif  // _LIBCPP_NO_EXCEPTIONS
@@ -2908,11 +2911,11 @@ valarray<_Tp>::valarray(const gslice_array<value_type>& __ga)
     : __begin_(0),
       __end_(0)
 {
-    size_t __n = __ga.__1d_.size();
+    const size_t __n = __ga.__1d_.size();
     if (__n)
     {
         __begin_ = __end_ = static_cast<value_type*>(
-           _VSTD::__libcpp_allocate(__n * sizeof(value_type), __alignof(value_type)));
+           _VSTD::__libcpp_allocate(__n * sizeof(value_type), _LIBCPP_ALIGNOF(value_type)));
 #ifndef _LIBCPP_NO_EXCEPTIONS
         try
         {
@@ -2926,7 +2929,7 @@ valarray<_Tp>::valarray(const gslice_array<value_type>& __ga)
         }
         catch (...)
         {
-            __clear();
+            __clear(__n);
             throw;
         }
 #endif  // _LIBCPP_NO_EXCEPTIONS
@@ -2938,11 +2941,11 @@ valarray<_Tp>::valarray(const mask_array<value_type>& __ma)
     : __begin_(0),
       __end_(0)
 {
-    size_t __n = __ma.__1d_.size();
+    const size_t __n = __ma.__1d_.size();
     if (__n)
     {
         __begin_ = __end_ = static_cast<value_type*>(
-           _VSTD::__libcpp_allocate(__n * sizeof(value_type), __alignof(value_type)));
+           _VSTD::__libcpp_allocate(__n * sizeof(value_type), _LIBCPP_ALIGNOF(value_type)));
 #ifndef _LIBCPP_NO_EXCEPTIONS
         try
         {
@@ -2956,7 +2959,7 @@ valarray<_Tp>::valarray(const mask_array<value_type>& __ma)
         }
         catch (...)
         {
-            __clear();
+            __clear(__n);
             throw;
         }
 #endif  // _LIBCPP_NO_EXCEPTIONS
@@ -2968,11 +2971,11 @@ valarray<_Tp>::valarray(const indirect_array<value_type>& __ia)
     : __begin_(0),
       __end_(0)
 {
-    size_t __n = __ia.__1d_.size();
+    const size_t __n = __ia.__1d_.size();
     if (__n)
     {
         __begin_ = __end_ = static_cast<value_type*>(
-           _VSTD::__libcpp_allocate(__n * sizeof(value_type), __alignof(value_type)));
+           _VSTD::__libcpp_allocate(__n * sizeof(value_type), _LIBCPP_ALIGNOF(value_type)));
 #ifndef _LIBCPP_NO_EXCEPTIONS
         try
         {
@@ -2986,7 +2989,7 @@ valarray<_Tp>::valarray(const indirect_array<value_type>& __ia)
         }
         catch (...)
         {
-            __clear();
+            __clear(__n);
             throw;
         }
 #endif  // _LIBCPP_NO_EXCEPTIONS
@@ -2997,7 +3000,7 @@ template <class _Tp>
 inline
 valarray<_Tp>::~valarray()
 {
-    __clear();
+    __clear(size());
 }
 
 template <class _Tp>
@@ -3007,9 +3010,9 @@ valarray<_Tp>::__assign_range(const value_type* __f, const value_type* __l)
     size_t __n = __l - __f;
     if (size() != __n)
     {
-        __clear();
+        __clear(size());
         __begin_ = static_cast<value_type*>(
-           _VSTD::__libcpp_allocate(__n * sizeof(value_type), __alignof(value_type)));
+           _VSTD::__libcpp_allocate(__n * sizeof(value_type), _LIBCPP_ALIGNOF(value_type)));
         __end_ = __begin_ + __n;
         _VSTD::uninitialized_copy(__f, __l, __begin_);
     } else {
@@ -3034,7 +3037,7 @@ inline
 valarray<_Tp>&
 valarray<_Tp>::operator=(valarray&& __v) _NOEXCEPT
 {
-    __clear();
+    __clear(size());
     __begin_ = __v.__begin_;
     __end_ = __v.__end_;
     __v.__begin_ = nullptr;
@@ -3265,7 +3268,7 @@ valarray<_Tp>::operator+() const
         __r.__begin_ =
             __r.__end_ =
                 static_cast<value_type*>(
-           _VSTD::__libcpp_allocate(__n * sizeof(value_type), __alignof(value_type)));
+           _VSTD::__libcpp_allocate(__n * sizeof(value_type), _LIBCPP_ALIGNOF(value_type)));
         for (const value_type* __p = __begin_; __n; ++__r.__end_, ++__p, --__n)
             ::new (__r.__end_) value_type(+*__p);
     }
@@ -3283,7 +3286,7 @@ valarray<_Tp>::operator-() const
         __r.__begin_ =
             __r.__end_ =
                 static_cast<value_type*>(
-           _VSTD::__libcpp_allocate(__n * sizeof(value_type), __alignof(value_type)));
+           _VSTD::__libcpp_allocate(__n * sizeof(value_type), _LIBCPP_ALIGNOF(value_type)));
         for (const value_type* __p = __begin_; __n; ++__r.__end_, ++__p, --__n)
             ::new (__r.__end_) value_type(-*__p);
     }
@@ -3301,7 +3304,7 @@ valarray<_Tp>::operator~() const
         __r.__begin_ =
             __r.__end_ =
                 static_cast<value_type*>(
-           _VSTD::__libcpp_allocate(__n * sizeof(value_type), __alignof(value_type)));
+           _VSTD::__libcpp_allocate(__n * sizeof(value_type), _LIBCPP_ALIGNOF(value_type)));
         for (const value_type* __p = __begin_; __n; ++__r.__end_, ++__p, --__n)
             ::new (__r.__end_) value_type(~*__p);
     }
@@ -3318,7 +3321,7 @@ valarray<_Tp>::operator!() const
     {
         __r.__begin_ =
             __r.__end_ =
-                static_cast<bool*>(_VSTD::__libcpp_allocate(__n * sizeof(bool), __alignof(bool)));
+                static_cast<bool*>(_VSTD::__libcpp_allocate(__n * sizeof(bool), _LIBCPP_ALIGNOF(bool)));
         for (const value_type* __p = __begin_; __n; ++__r.__end_, ++__p, --__n)
             ::new (__r.__end_) bool(!*__p);
     }
@@ -3639,7 +3642,7 @@ valarray<_Tp>::shift(int __i) const
         __r.__begin_ =
             __r.__end_ =
                 static_cast<value_type*>(
-           _VSTD::__libcpp_allocate(__n * sizeof(value_type), __alignof(value_type)));
+           _VSTD::__libcpp_allocate(__n * sizeof(value_type), _LIBCPP_ALIGNOF(value_type)));
         const value_type* __sb;
         value_type* __tb;
         value_type* __te;
@@ -3678,7 +3681,7 @@ valarray<_Tp>::cshift(int __i) const
         __r.__begin_ =
             __r.__end_ =
                 static_cast<value_type*>(
-           _VSTD::__libcpp_allocate(__n * sizeof(value_type), __alignof(value_type)));
+           _VSTD::__libcpp_allocate(__n * sizeof(value_type), _LIBCPP_ALIGNOF(value_type)));
         __i %= static_cast<int>(__n);
         const value_type* __m = __i >= 0 ? __begin_ + __i : __end_ + __i;
         for (const value_type* __s = __m; __s != __end_; ++__r.__end_, ++__s)
@@ -3700,7 +3703,7 @@ valarray<_Tp>::apply(value_type __f(value_type)) const
         __r.__begin_ =
             __r.__end_ =
                 static_cast<value_type*>(
-           _VSTD::__libcpp_allocate(__n * sizeof(value_type), __alignof(value_type)));
+           _VSTD::__libcpp_allocate(__n * sizeof(value_type), _LIBCPP_ALIGNOF(value_type)));
         for (const value_type* __p = __begin_; __n; ++__r.__end_, ++__p, --__n)
             ::new (__r.__end_) value_type(__f(*__p));
     }
@@ -3718,7 +3721,7 @@ valarray<_Tp>::apply(value_type __f(const value_type&)) const
         __r.__begin_ =
             __r.__end_ =
                 static_cast<value_type*>(
-           _VSTD::__libcpp_allocate(__n * sizeof(value_type), __alignof(value_type)));
+           _VSTD::__libcpp_allocate(__n * sizeof(value_type), _LIBCPP_ALIGNOF(value_type)));
         for (const value_type* __p = __begin_; __n; ++__r.__end_, ++__p, --__n)
             ::new (__r.__end_) value_type(__f(*__p));
     }
@@ -3726,38 +3729,38 @@ valarray<_Tp>::apply(value_type __f(const value_type&)) const
 }
 
 template <class _Tp>
-void
-valarray<_Tp>::__clear()
+inline
+void valarray<_Tp>::__clear(size_t __capacity)
 {
-    if (__begin_ != nullptr)
-    {
-        while (__end_ != __begin_)
-            (--__end_)->~value_type();
-        _VSTD::__libcpp_deallocate(__begin_, __alignof(value_type));
-        __begin_ = __end_ = nullptr;
-    }
+  if (__begin_ != nullptr)
+  {
+    while (__end_ != __begin_)
+      (--__end_)->~value_type();
+    _VSTD::__libcpp_deallocate(__begin_, __capacity * sizeof(value_type), _LIBCPP_ALIGNOF(value_type));
+    __begin_ = __end_ = nullptr;
+  }
 }
 
 template <class _Tp>
 void
 valarray<_Tp>::resize(size_t __n, value_type __x)
 {
-    __clear();
+    __clear(size());
     if (__n)
     {
         __begin_ = __end_ = static_cast<value_type*>(
-           _VSTD::__libcpp_allocate(__n * sizeof(value_type), __alignof(value_type)));
+           _VSTD::__libcpp_allocate(__n * sizeof(value_type), _LIBCPP_ALIGNOF(value_type)));
 #ifndef _LIBCPP_NO_EXCEPTIONS
         try
         {
 #endif  // _LIBCPP_NO_EXCEPTIONS
-            for (; __n; --__n, ++__end_)
+            for (size_t __n_left = __n; __n_left; --__n_left, ++__end_)
                 ::new (__end_) value_type(__x);
 #ifndef _LIBCPP_NO_EXCEPTIONS
         }
         catch (...)
         {
-            __clear();
+            __clear(__n);
             throw;
         }
 #endif  // _LIBCPP_NO_EXCEPTIONS
diff --git a/include/variant b/include/variant
index f9098f42249a..a4339de6cde4 100644
--- a/include/variant
+++ b/include/variant
@@ -23,8 +23,8 @@ namespace std {
 
     // 20.7.2.1, constructors
     constexpr variant() noexcept(see below);
-    variant(const variant&);
-    variant(variant&&) noexcept(see below);
+    variant(const variant&);                // constexpr in C++20
+    variant(variant&&) noexcept(see below); // constexpr in C++20
 
     template <class T> constexpr variant(T&&) noexcept(see below);
 
@@ -46,8 +46,8 @@ namespace std {
     ~variant();
 
     // 20.7.2.3, assignment
-    variant& operator=(const variant&);
-    variant& operator=(variant&&) noexcept(see below);
+    variant& operator=(const variant&);                // constexpr in C++20
+    variant& operator=(variant&&) noexcept(see below); // constexpr in C++20
 
     template <class T> variant& operator=(T&&) noexcept(see below);
 
@@ -208,6 +208,7 @@ namespace std {
 #include <type_traits>
 #include <utility>
 #include <limits>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
@@ -218,7 +219,7 @@ _LIBCPP_PUSH_MACROS
 
 namespace std { // explicitly not using versioning namespace
 
-class _LIBCPP_EXCEPTION_ABI bad_variant_access : public exception {
+class _LIBCPP_EXCEPTION_ABI _LIBCPP_AVAILABILITY_BAD_VARIANT_ACCESS bad_variant_access : public exception {
 public:
   virtual const char* what() const _NOEXCEPT;
 };
@@ -231,6 +232,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 _LIBCPP_NORETURN
 inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS
 void __throw_bad_variant_access() {
 #ifndef _LIBCPP_NO_EXCEPTIONS
         throw bad_variant_access();
@@ -1064,7 +1066,7 @@ public:
 #ifndef _LIBCPP_NO_EXCEPTIONS
       // EXTENSION: When the move construction of `__lhs` into `__rhs` throws
       // and `__tmp` is nothrow move constructible then we move `__tmp` back
-      // into `__rhs` and provide the strong exception safety guarentee.
+      // into `__rhs` and provide the strong exception safety guarantee.
       try {
         this->__generic_construct(*__rhs, _VSTD::move(*__lhs));
       } catch (...) {
@@ -1320,7 +1322,8 @@ constexpr bool holds_alternative(const variant<_Types...>& __v) noexcept {
 
 template <size_t _Ip, class _Vp>
 inline _LIBCPP_INLINE_VISIBILITY
-static constexpr auto&& __generic_get(_Vp&& __v) {
+_LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS
+constexpr auto&& __generic_get(_Vp&& __v) {
   using __variant_detail::__access::__variant;
   if (!__holds_alternative<_Ip>(__v)) {
     __throw_bad_variant_access();
@@ -1330,6 +1333,7 @@ static constexpr auto&& __generic_get(_Vp&& __v) {
 
 template <size_t _Ip, class... _Types>
 inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS
 constexpr variant_alternative_t<_Ip, variant<_Types...>>& get(
     variant<_Types...>& __v) {
   static_assert(_Ip < sizeof...(_Types));
@@ -1339,6 +1343,7 @@ constexpr variant_alternative_t<_Ip, variant<_Types...>>& get(
 
 template <size_t _Ip, class... _Types>
 inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS
 constexpr variant_alternative_t<_Ip, variant<_Types...>>&& get(
     variant<_Types...>&& __v) {
   static_assert(_Ip < sizeof...(_Types));
@@ -1348,6 +1353,7 @@ constexpr variant_alternative_t<_Ip, variant<_Types...>>&& get(
 
 template <size_t _Ip, class... _Types>
 inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS
 constexpr const variant_alternative_t<_Ip, variant<_Types...>>& get(
     const variant<_Types...>& __v) {
   static_assert(_Ip < sizeof...(_Types));
@@ -1357,6 +1363,7 @@ constexpr const variant_alternative_t<_Ip, variant<_Types...>>& get(
 
 template <size_t _Ip, class... _Types>
 inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS
 constexpr const variant_alternative_t<_Ip, variant<_Types...>>&& get(
     const variant<_Types...>&& __v) {
   static_assert(_Ip < sizeof...(_Types));
@@ -1366,6 +1373,7 @@ constexpr const variant_alternative_t<_Ip, variant<_Types...>>&& get(
 
 template <class _Tp, class... _Types>
 inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS
 constexpr _Tp& get(variant<_Types...>& __v) {
   static_assert(!is_void_v<_Tp>);
   return _VSTD::get<__find_exactly_one_t<_Tp, _Types...>::value>(__v);
@@ -1373,6 +1381,7 @@ constexpr _Tp& get(variant<_Types...>& __v) {
 
 template <class _Tp, class... _Types>
 inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS
 constexpr _Tp&& get(variant<_Types...>&& __v) {
   static_assert(!is_void_v<_Tp>);
   return _VSTD::get<__find_exactly_one_t<_Tp, _Types...>::value>(
@@ -1381,6 +1390,7 @@ constexpr _Tp&& get(variant<_Types...>&& __v) {
 
 template <class _Tp, class... _Types>
 inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS
 constexpr const _Tp& get(const variant<_Types...>& __v) {
   static_assert(!is_void_v<_Tp>);
   return _VSTD::get<__find_exactly_one_t<_Tp, _Types...>::value>(__v);
@@ -1388,6 +1398,7 @@ constexpr const _Tp& get(const variant<_Types...>& __v) {
 
 template <class _Tp, class... _Types>
 inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS
 constexpr const _Tp&& get(const variant<_Types...>&& __v) {
   static_assert(!is_void_v<_Tp>);
   return _VSTD::get<__find_exactly_one_t<_Tp, _Types...>::value>(
@@ -1437,6 +1448,16 @@ get_if(const variant<_Types...>* __v) noexcept {
   return _VSTD::get_if<__find_exactly_one_t<_Tp, _Types...>::value>(__v);
 }
 
+template <class _Operator>
+struct __convert_to_bool {
+  template <class _T1, class _T2>
+  _LIBCPP_INLINE_VISIBILITY constexpr bool operator()(_T1 && __t1, _T2&& __t2) const {
+    static_assert(std::is_convertible<decltype(_Operator{}(_VSTD::forward<_T1>(__t1), _VSTD::forward<_T2>(__t2))), bool>::value,
+        "the relational operator does not return a type which is implicitly convertible to bool");
+    return _Operator{}(_VSTD::forward<_T1>(__t1), _VSTD::forward<_T2>(__t2));
+  }
+};
+
 template <class... _Types>
 inline _LIBCPP_INLINE_VISIBILITY
 constexpr bool operator==(const variant<_Types...>& __lhs,
@@ -1444,7 +1465,7 @@ constexpr bool operator==(const variant<_Types...>& __lhs,
   using __variant_detail::__visitation::__variant;
   if (__lhs.index() != __rhs.index()) return false;
   if (__lhs.valueless_by_exception()) return true;
-  return __variant::__visit_value_at(__lhs.index(), equal_to<>{}, __lhs, __rhs);
+  return __variant::__visit_value_at(__lhs.index(), __convert_to_bool<equal_to<>>{}, __lhs, __rhs);
 }
 
 template <class... _Types>
@@ -1455,7 +1476,7 @@ constexpr bool operator!=(const variant<_Types...>& __lhs,
   if (__lhs.index() != __rhs.index()) return true;
   if (__lhs.valueless_by_exception()) return false;
   return __variant::__visit_value_at(
-      __lhs.index(), not_equal_to<>{}, __lhs, __rhs);
+      __lhs.index(), __convert_to_bool<not_equal_to<>>{}, __lhs, __rhs);
 }
 
 template <class... _Types>
@@ -1467,7 +1488,7 @@ constexpr bool operator<(const variant<_Types...>& __lhs,
   if (__lhs.valueless_by_exception()) return true;
   if (__lhs.index() < __rhs.index()) return true;
   if (__lhs.index() > __rhs.index()) return false;
-  return __variant::__visit_value_at(__lhs.index(), less<>{}, __lhs, __rhs);
+  return __variant::__visit_value_at(__lhs.index(), __convert_to_bool<less<>>{}, __lhs, __rhs);
 }
 
 template <class... _Types>
@@ -1479,7 +1500,7 @@ constexpr bool operator>(const variant<_Types...>& __lhs,
   if (__rhs.valueless_by_exception()) return true;
   if (__lhs.index() > __rhs.index()) return true;
   if (__lhs.index() < __rhs.index()) return false;
-  return __variant::__visit_value_at(__lhs.index(), greater<>{}, __lhs, __rhs);
+  return __variant::__visit_value_at(__lhs.index(), __convert_to_bool<greater<>>{}, __lhs, __rhs);
 }
 
 template <class... _Types>
@@ -1492,7 +1513,7 @@ constexpr bool operator<=(const variant<_Types...>& __lhs,
   if (__lhs.index() < __rhs.index()) return true;
   if (__lhs.index() > __rhs.index()) return false;
   return __variant::__visit_value_at(
-      __lhs.index(), less_equal<>{}, __lhs, __rhs);
+      __lhs.index(), __convert_to_bool<less_equal<>>{}, __lhs, __rhs);
 }
 
 template <class... _Types>
@@ -1505,11 +1526,12 @@ constexpr bool operator>=(const variant<_Types...>& __lhs,
   if (__lhs.index() > __rhs.index()) return true;
   if (__lhs.index() < __rhs.index()) return false;
   return __variant::__visit_value_at(
-      __lhs.index(), greater_equal<>{}, __lhs, __rhs);
+      __lhs.index(), __convert_to_bool<greater_equal<>>{}, __lhs, __rhs);
 }
 
 template <class _Visitor, class... _Vs>
 inline _LIBCPP_INLINE_VISIBILITY
+_LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS
 constexpr decltype(auto) visit(_Visitor&& __visitor, _Vs&&... __vs) {
   using __variant_detail::__visitation::__variant;
   bool __results[] = {__vs.valueless_by_exception()...};
diff --git a/include/vector b/include/vector
index 0f5006f37567..edb6d3e09f5f 100644
--- a/include/vector
+++ b/include/vector
@@ -261,6 +261,11 @@ template <class T, class Allocator>
 void swap(vector<T,Allocator>& x, vector<T,Allocator>& y)
     noexcept(noexcept(x.swap(y)));
 
+template <class T, class Allocator, class U>
+    void erase(vector<T, Allocator>& c, const U& value);       // C++20
+template <class T, class Allocator, class Predicate>
+    void erase_if(vector<T, Allocator>& c, Predicate pred);    // C++20
+
 }  // std
 
 */
@@ -276,6 +281,7 @@ void swap(vector<T,Allocator>& x, vector<T,Allocator>& y)
 #include <stdexcept>
 #include <algorithm>
 #include <cstring>
+#include <version>
 #include <__split_buffer>
 #include <__functional_base>
 
@@ -540,13 +546,14 @@ public:
                                     value_type,
                                     typename iterator_traits<_ForwardIterator>::reference>::value>::type* = 0);
 
-#if _LIBCPP_DEBUG_LEVEL >= 2
     _LIBCPP_INLINE_VISIBILITY
     ~vector()
     {
+        __annotate_delete();
+#if _LIBCPP_DEBUG_LEVEL >= 2
         __get_db()->__erase_c(this);
-    }
 #endif
+    }
 
     vector(const vector& __x);
     vector(const vector& __x, const allocator_type& __a);
@@ -2453,7 +2460,7 @@ private:
     void __vdeallocate() _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY
     static size_type __align_it(size_type __new_size) _NOEXCEPT
-        {return __new_size + (__bits_per_word-1) & ~((size_type)__bits_per_word-1);};
+        {return __new_size + (__bits_per_word-1) & ~((size_type)__bits_per_word-1);}
     _LIBCPP_INLINE_VISIBILITY  size_type __recommend(size_type __new_size) const;
     _LIBCPP_INLINE_VISIBILITY void __construct_at_end(size_type __n, bool __x);
     template <class _ForwardIterator>
@@ -2604,6 +2611,13 @@ vector<bool, _Allocator>::__construct_at_end(size_type __n, bool __x)
 {
     size_type __old_size = this->__size_;
     this->__size_ += __n;
+    if (__old_size == 0 || ((__old_size - 1) / __bits_per_word) != ((this->__size_ - 1) / __bits_per_word))
+    {
+        if (this->__size_ <= __bits_per_word)
+            this->__begin_[0] = __storage_type(0);
+        else
+            this->__begin_[(this->__size_ - 1) / __bits_per_word] = __storage_type(0);
+    }
     _VSTD::fill_n(__make_iter(__old_size), __n, __x);
 }
 
@@ -2618,6 +2632,13 @@ vector<bool, _Allocator>::__construct_at_end(_ForwardIterator __first, _ForwardI
 {
     size_type __old_size = this->__size_;
     this->__size_ += _VSTD::distance(__first, __last);
+    if (__old_size == 0 || ((__old_size - 1) / __bits_per_word) != ((this->__size_ - 1) / __bits_per_word))
+    {
+        if (this->__size_ <= __bits_per_word)
+            this->__begin_[0] = __storage_type(0);
+        else
+            this->__begin_[(this->__size_ - 1) / __bits_per_word] = __storage_type(0);
+    }
     _VSTD::copy(__first, __last, __make_iter(__old_size));
 }
 
@@ -3392,6 +3413,18 @@ swap(vector<_Tp, _Allocator>& __x, vector<_Tp, _Allocator>& __y)
     __x.swap(__y);
 }
 
+#if _LIBCPP_STD_VER > 17
+template <class _Tp, class _Allocator, class _Up>
+inline _LIBCPP_INLINE_VISIBILITY
+void erase(vector<_Tp, _Allocator>& __c, const _Up& __v)
+{ __c.erase(_VSTD::remove(__c.begin(), __c.end(), __v), __c.end()); }
+
+template <class _Tp, class _Allocator, class _Predicate>
+inline _LIBCPP_INLINE_VISIBILITY
+void erase_if(vector<_Tp, _Allocator>& __c, _Predicate __pred)
+{ __c.erase(_VSTD::remove_if(__c.begin(), __c.end(), __pred), __c.end()); }
+#endif
+
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
diff --git a/include/version b/include/version
index 23a872ea85ac..e37afc448601 100644
--- a/include/version
+++ b/include/version
@@ -12,7 +12,105 @@
 #define _LIBCPP_VERSIONH
 
 /*
-    version synopsis
+  version synopsis
+
+Macro name                                              Value   Headers
+__cpp_lib_addressof_constexpr                           201603L <memory>
+__cpp_lib_allocator_traits_is_always_equal              201411L <memory> <scoped_allocator> <string>
+                                                                <deque> <forward_list> <list>
+                                                                <vector> <map> <set>
+                                                                <unordered_map> <unordered_set>
+__cpp_lib_any                                           201606L <any>
+__cpp_lib_apply                                         201603L <tuple>
+__cpp_lib_array_constexpr                               201603L <iterator> <array>
+__cpp_lib_as_const                                      201510L <utility>
+__cpp_lib_atomic_is_always_lock_free                    201603L <atomic>
+__cpp_lib_atomic_ref                                    201806L <atomic>
+__cpp_lib_bind_front                                    201811L <functional>
+__cpp_lib_bit_cast                                      201806L <bit>
+__cpp_lib_bool_constant                                 201505L <type_traits>
+__cpp_lib_boyer_moore_searcher                          201603L <functional>
+__cpp_lib_byte                                          201603L <cstddef>
+__cpp_lib_char8_t                                       201811L <atomic> <filesystem> <istream>
+                                                                <limits> <locale> <ostream>
+                                                                <string> <string_view>
+__cpp_lib_chrono                                        201611L <chrono>
+__cpp_lib_chrono_udls                                   201304L <chrono>
+__cpp_lib_clamp                                         201603L <algorithm>
+__cpp_lib_complex_udls                                  201309L <complex>
+__cpp_lib_concepts                                      201806L <concepts>
+__cpp_lib_constexpr_misc                                201811L <array> <functional> <iterator>
+                                                                <string_view> <tuple> <utility>
+__cpp_lib_constexpr_swap_algorithms                     201806L <algorithm>
+__cpp_lib_destroying_delete                             201806L <new>
+__cpp_lib_enable_shared_from_this                       201603L <memory>
+__cpp_lib_erase_if                                      201811L <string> <deque> <forward_list>
+                                                                <list> <vector> <map>
+                                                                <set> <unordered_map> <unordered_set>
+__cpp_lib_exchange_function                             201304L <utility>
+__cpp_lib_execution                                     201603L <execution>
+__cpp_lib_filesystem                                    201703L <filesystem>
+__cpp_lib_gcd_lcm                                       201606L <numeric>
+__cpp_lib_generic_associative_lookup                    201304L <map> <set>
+__cpp_lib_generic_unordered_lookup                      201811L <unordered_map> <unordered_set>
+__cpp_lib_hardware_interference_size                    201703L <new>
+__cpp_lib_has_unique_object_representations             201606L <type_traits>
+__cpp_lib_hypot                                         201603L <cmath>
+__cpp_lib_incomplete_container_elements                 201505L <forward_list> <list> <vector>
+__cpp_lib_integer_sequence                              201304L <utility>
+__cpp_lib_integral_constant_callable                    201304L <type_traits>
+__cpp_lib_invoke                                        201411L <functional>
+__cpp_lib_is_aggregate                                  201703L <type_traits>
+__cpp_lib_is_constant_evaluated                         201811L <type_traits>
+__cpp_lib_is_final                                      201402L <type_traits>
+__cpp_lib_is_invocable                                  201703L <type_traits>
+__cpp_lib_is_null_pointer                               201309L <type_traits>
+__cpp_lib_is_swappable                                  201603L <type_traits>
+__cpp_lib_launder                                       201606L <new>
+__cpp_lib_list_remove_return_type                       201806L <forward_list> <list>
+__cpp_lib_logical_traits                                201510L <type_traits>
+__cpp_lib_make_from_tuple                               201606L <tuple>
+__cpp_lib_make_reverse_iterator                         201402L <iterator>
+__cpp_lib_make_unique                                   201304L <memory>
+__cpp_lib_map_try_emplace                               201411L <map>
+__cpp_lib_math_special_functions                        201603L <cmath>
+__cpp_lib_memory_resource                               201603L <memory_resource>
+__cpp_lib_node_extract                                  201606L <map> <set> <unordered_map>
+                                                                <unordered_set>
+__cpp_lib_nonmember_container_access                    201411L <iterator> <array> <deque>
+                                                                <forward_list> <list> <map>
+                                                                <regex> <set> <string>
+                                                                <unordered_map> <unordered_set> <vector>
+__cpp_lib_not_fn                                        201603L <functional>
+__cpp_lib_null_iterators                                201304L <iterator>
+__cpp_lib_optional                                      201606L <optional>
+__cpp_lib_parallel_algorithm                            201603L <algorithm> <numeric>
+__cpp_lib_quoted_string_io                              201304L <iomanip>
+__cpp_lib_ranges                                        201811L <algorithm> <functional> <iterator>
+                                                                <memory> <ranges>
+__cpp_lib_raw_memory_algorithms                         201606L <memory>
+__cpp_lib_result_of_sfinae                              201210L <functional> <type_traits>
+__cpp_lib_robust_nonmodifying_seq_ops                   201304L <algorithm>
+__cpp_lib_sample                                        201603L <algorithm>
+__cpp_lib_scoped_lock                                   201703L <mutex>
+__cpp_lib_shared_mutex                                  201505L <shared_mutex>
+__cpp_lib_shared_ptr_arrays                             201611L <memory>
+__cpp_lib_shared_ptr_weak_type                          201606L <memory>
+__cpp_lib_shared_timed_mutex                            201402L <shared_mutex>
+__cpp_lib_string_udls                                   201304L <string>
+__cpp_lib_string_view                                   201606L <string> <string_view>
+__cpp_lib_three_way_comparison                          201711L <compare>
+__cpp_lib_to_chars                                      201611L <utility>
+__cpp_lib_transformation_trait_aliases                  201304L <type_traits>
+__cpp_lib_transparent_operators                         201510L <functional>
+                                                        201210L // C++14
+__cpp_lib_tuple_element_t                               201402L <tuple>
+__cpp_lib_tuples_by_type                                201304L <utility> <tuple>
+__cpp_lib_type_trait_variable_templates                 201510L <type_traits>
+__cpp_lib_uncaught_exceptions                           201411L <exception>
+__cpp_lib_unordered_map_try_emplace                     201411L <unordered_map>
+__cpp_lib_variant                                       201606L <variant>
+__cpp_lib_void_t                                        201411L <type_traits>
 
 */
 
@@ -22,4 +120,113 @@
 #pragma GCC system_header
 #endif
 
-#endif  // _LIBCPP_VERSIONH
+#if _LIBCPP_STD_VER > 11
+# define __cpp_lib_chrono_udls                          201304L
+# define __cpp_lib_complex_udls                         201309L
+# define __cpp_lib_exchange_function                    201304L
+# define __cpp_lib_generic_associative_lookup           201304L
+# define __cpp_lib_integer_sequence                     201304L
+# define __cpp_lib_integral_constant_callable           201304L
+# define __cpp_lib_is_final                             201402L
+# define __cpp_lib_is_null_pointer                      201309L
+# define __cpp_lib_make_reverse_iterator                201402L
+# define __cpp_lib_make_unique                          201304L
+# define __cpp_lib_null_iterators                       201304L
+# define __cpp_lib_quoted_string_io                     201304L
+# define __cpp_lib_result_of_sfinae                     201210L
+# define __cpp_lib_robust_nonmodifying_seq_ops          201304L
+# if !defined(_LIBCPP_HAS_NO_THREADS)
+#   define __cpp_lib_shared_timed_mutex                 201402L
+# endif
+# define __cpp_lib_string_udls                          201304L
+# define __cpp_lib_transformation_trait_aliases         201304L
+# define __cpp_lib_transparent_operators                201210L
+# define __cpp_lib_tuple_element_t                      201402L
+# define __cpp_lib_tuples_by_type                       201304L
+#endif
+
+#if _LIBCPP_STD_VER > 14
+# if !defined(_LIBCPP_HAS_NO_BUILTIN_ADDRESSOF)
+#   define __cpp_lib_addressof_constexpr                201603L
+# endif
+# define __cpp_lib_allocator_traits_is_always_equal     201411L
+# define __cpp_lib_any                                  201606L
+# define __cpp_lib_apply                                201603L
+# define __cpp_lib_array_constexpr                      201603L
+# define __cpp_lib_as_const                             201510L
+# if !defined(_LIBCPP_HAS_NO_THREADS)
+#   define __cpp_lib_atomic_is_always_lock_free         201603L
+# endif
+# define __cpp_lib_bool_constant                        201505L
+// # define __cpp_lib_boyer_moore_searcher                 201603L
+# define __cpp_lib_byte                                 201603L
+# define __cpp_lib_chrono                               201611L
+# define __cpp_lib_clamp                                201603L
+# define __cpp_lib_enable_shared_from_this              201603L
+// # define __cpp_lib_execution                            201603L
+# define __cpp_lib_filesystem                           201703L
+# define __cpp_lib_gcd_lcm                              201606L
+# define __cpp_lib_hardware_interference_size           201703L
+# if defined(_LIBCPP_HAS_UNIQUE_OBJECT_REPRESENTATIONS)
+#   define __cpp_lib_has_unique_object_representations  201606L
+# endif
+# define __cpp_lib_hypot                                201603L
+# define __cpp_lib_incomplete_container_elements        201505L
+# define __cpp_lib_invoke                               201411L
+# if !defined(_LIBCPP_HAS_NO_IS_AGGREGATE)
+#   define __cpp_lib_is_aggregate                       201703L
+# endif
+# define __cpp_lib_is_invocable                         201703L
+# define __cpp_lib_is_swappable                         201603L
+# define __cpp_lib_launder                              201606L
+# define __cpp_lib_logical_traits                       201510L
+# define __cpp_lib_make_from_tuple                      201606L
+# define __cpp_lib_map_try_emplace                      201411L
+// # define __cpp_lib_math_special_functions               201603L
+// # define __cpp_lib_memory_resource                      201603L
+# define __cpp_lib_node_extract                         201606L
+# define __cpp_lib_nonmember_container_access           201411L
+# define __cpp_lib_not_fn                               201603L
+# define __cpp_lib_optional                             201606L
+// # define __cpp_lib_parallel_algorithm                   201603L
+# define __cpp_lib_raw_memory_algorithms                201606L
+# define __cpp_lib_sample                               201603L
+# define __cpp_lib_scoped_lock                          201703L
+# if !defined(_LIBCPP_HAS_NO_THREADS)
+#   define __cpp_lib_shared_mutex                       201505L
+# endif
+// # define __cpp_lib_shared_ptr_arrays                    201611L
+# define __cpp_lib_shared_ptr_weak_type                 201606L
+# define __cpp_lib_string_view                          201606L
+// # define __cpp_lib_to_chars                             201611L
+# undef  __cpp_lib_transparent_operators
+# define __cpp_lib_transparent_operators                201510L
+# define __cpp_lib_type_trait_variable_templates        201510L
+# define __cpp_lib_uncaught_exceptions                  201411L
+# define __cpp_lib_unordered_map_try_emplace            201411L
+# define __cpp_lib_variant                              201606L
+# define __cpp_lib_void_t                               201411L
+#endif
+
+#if _LIBCPP_STD_VER > 17
+# if !defined(_LIBCPP_HAS_NO_THREADS)
+// #   define __cpp_lib_atomic_ref                         201806L
+# endif
+// # define __cpp_lib_bind_front                           201811L
+// # define __cpp_lib_bit_cast                             201806L
+# if !defined(_LIBCPP_NO_HAS_CHAR8_T)
+#   define __cpp_lib_char8_t                            201811L
+# endif
+// # define __cpp_lib_concepts                             201806L
+// # define __cpp_lib_constexpr_misc                       201811L
+// # define __cpp_lib_constexpr_swap_algorithms            201806L
+// # define __cpp_lib_destroying_delete                    201806L
+# define __cpp_lib_erase_if                             201811L
+// # define __cpp_lib_generic_unordered_lookup             201811L
+// # define __cpp_lib_is_constant_evaluated                201811L
+// # define __cpp_lib_list_remove_return_type              201806L
+// # define __cpp_lib_ranges                               201811L
+// # define __cpp_lib_three_way_comparison                 201711L
+#endif
+
+#endif // _LIBCPP_VERSIONH