diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -205,8 +205,8 @@
 #  define _LIBCPP_COMPILER_IBM
 #endif
 
-#if defined(_LIBCPP_COMPILER_GCC) && __cplusplus < 201103L
-#error "libc++ does not support using GCC with C++03. Please enable C++11"
+#if !defined(_LIBCPP_COMPILER_CLANG_BASED) && __cplusplus < 201103L
+#  error "libc++ does not support compilers other than Clang in C++03. Please enable C++11."
 #endif
 
 // FIXME: ABI detection should be done via compiler builtin macros. This
@@ -387,56 +387,47 @@
 #endif
 
 #ifndef _LIBCPP_CXX03_LANG
-# define _LIBCPP_ALIGNOF(_Tp) alignof(_Tp)
-#elif defined(_LIBCPP_COMPILER_CLANG_BASED)
-# define _LIBCPP_ALIGNOF(_Tp) _Alignof(_Tp)
-#else
-# error "We don't know a correct way to implement alignof(T) in C++03 outside of Clang"
-#endif
-
-#define _LIBCPP_PREFERRED_ALIGNOF(_Tp) __alignof(_Tp)
-
-#if defined(_LIBCPP_COMPILER_CLANG_BASED)
-
-#if defined(__APPLE__) && !defined(__i386__) && !defined(__x86_64__) &&       \
-    (!defined(__arm__) || __ARM_ARCH_7K__ >= 2)
-#  define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
-#endif
 
-#if __has_feature(cxx_alignas)
+#  define _LIBCPP_ALIGNOF(_Tp) alignof(_Tp)
 #  define _ALIGNAS_TYPE(x) alignas(x)
 #  define _ALIGNAS(x) alignas(x)
+#  define _LIBCPP_NORETURN [[noreturn]]
+#  define _NOEXCEPT noexcept
+#  define _NOEXCEPT_(x) noexcept(x)
+
 #else
+
+#  define _LIBCPP_ALIGNOF(_Tp) _Alignof(_Tp)
 #  define _ALIGNAS_TYPE(x) __attribute__((__aligned__(_LIBCPP_ALIGNOF(x))))
 #  define _ALIGNAS(x) __attribute__((__aligned__(x)))
-#endif
+#  define _LIBCPP_NORETURN __attribute__ ((noreturn))
+#  define nullptr __nullptr
+#  define _NOEXCEPT throw()
+#  define _NOEXCEPT_(x)
 
-#if __cplusplus < 201103L
-typedef __char16_t char16_t;
-typedef __char32_t char32_t;
-#endif
+using char16_t = __char16_t;
+using char32_t = __char32_t;
 
-#if !__has_feature(cxx_exceptions)
-#  define _LIBCPP_NO_EXCEPTIONS
 #endif
 
-#if __has_feature(cxx_attributes)
-#  define _LIBCPP_NORETURN [[noreturn]]
-#else
-#  define _LIBCPP_NORETURN __attribute__ ((noreturn))
+#if !defined(__cpp_exceptions) || __cpp_exceptions < 199711L
+#    define _LIBCPP_NO_EXCEPTIONS
 #endif
 
-#ifdef _LIBCPP_CXX03_LANG
-# define nullptr __nullptr
-#endif
+#define _LIBCPP_PREFERRED_ALIGNOF(_Tp) __alignof(_Tp)
+
+#  if defined(__APPLE__) && !defined(__i386__) && !defined(__x86_64__) &&       \
+    (!defined(__arm__) || __ARM_ARCH_7K__ >= 2)
+#    define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
+#  endif
 
 // Objective-C++ features (opt-in)
 #if __has_feature(objc_arc)
-#define _LIBCPP_HAS_OBJC_ARC
+#  define _LIBCPP_HAS_OBJC_ARC
 #endif
 
 #if __has_feature(objc_arc_weak)
-#define _LIBCPP_HAS_OBJC_ARC_WEAK
+#  define _LIBCPP_HAS_OBJC_ARC_WEAK
 #endif
 
 #if __has_extension(blocks)
@@ -447,85 +438,60 @@
 #  define _LIBCPP_HAS_BLOCKS_RUNTIME
 #endif
 
-#if !(__has_feature(cxx_noexcept))
-#define _LIBCPP_HAS_NO_NOEXCEPT
-#endif
-
-#if !__has_feature(address_sanitizer)
-#define _LIBCPP_HAS_NO_ASAN
-#endif
-
 // Allow for build-time disabling of unsigned integer sanitization
 #if __has_attribute(no_sanitize)
-#define _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK __attribute__((__no_sanitize__("unsigned-integer-overflow")))
+#  define _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK __attribute__((__no_sanitize__("unsigned-integer-overflow")))
 #endif
 
-#define _LIBCPP_ALWAYS_INLINE __attribute__ ((__always_inline__))
-
-#define _LIBCPP_DISABLE_EXTENSION_WARNING __extension__
+#if defined(_LIBCPP_COMPILER_CLANG_BASED)
 
-#elif defined(_LIBCPP_COMPILER_GCC)
+#  if !__has_feature(address_sanitizer)
+#    define _LIBCPP_HAS_NO_ASAN
+#  endif
 
-#define _ALIGNAS(x) __attribute__((__aligned__(x)))
-#define _ALIGNAS_TYPE(x) __attribute__((__aligned__(_LIBCPP_ALIGNOF(x))))
+#  if !(__has_feature(cxx_noexcept))
+#    define _LIBCPP_HAS_NO_NOEXCEPT
+#  endif
 
-#define _LIBCPP_NORETURN __attribute__((noreturn))
+#  define _LIBCPP_ALWAYS_INLINE __attribute__ ((__always_inline__))
 
-#if !defined(__EXCEPTIONS)
-#  define _LIBCPP_NO_EXCEPTIONS
-#endif
+#  define _LIBCPP_DISABLE_EXTENSION_WARNING __extension__
 
-#if !defined(__SANITIZE_ADDRESS__)
-#define _LIBCPP_HAS_NO_ASAN
-#endif
+#elif defined(_LIBCPP_COMPILER_GCC)
 
-#define _LIBCPP_ALWAYS_INLINE __attribute__ ((__always_inline__))
+#  if !defined(__SANITIZE_ADDRESS__)
+#    define _LIBCPP_HAS_NO_ASAN
+#  endif
 
-#define _LIBCPP_DISABLE_EXTENSION_WARNING __extension__
+#  define _LIBCPP_ALWAYS_INLINE __attribute__ ((__always_inline__))
+#  define _LIBCPP_DISABLE_EXTENSION_WARNING __extension__
 
 #elif defined(_LIBCPP_COMPILER_MSVC)
 
-#define _LIBCPP_WARNING(x) __pragma(message(__FILE__ "(" _LIBCPP_TOSTRING(__LINE__) ") : warning note: " x))
-
-#if _MSC_VER < 1900
-#error "MSVC versions prior to Visual Studio 2015 are not supported"
-#endif
-
-#define __alignof__ __alignof
-#define _LIBCPP_NORETURN __declspec(noreturn)
-#define _ALIGNAS(x) __declspec(align(x))
-#define _ALIGNAS_TYPE(x) alignas(x)
-
-#define _LIBCPP_WEAK
-
-#define _LIBCPP_HAS_NO_ASAN
+#  define _LIBCPP_WARNING(x) __pragma(message(__FILE__ "(" _LIBCPP_TOSTRING(__LINE__) ") : warning note: " x))
 
-#define _LIBCPP_ALWAYS_INLINE __forceinline
-
-#define _LIBCPP_HAS_NO_VECTOR_EXTENSION
+#  if _MSC_VER < 1900
+#    error "MSVC versions prior to Visual Studio 2015 are not supported"
+#  endif
 
-#define _LIBCPP_DISABLE_EXTENSION_WARNING
+#  define _LIBCPP_HAS_NO_ASAN
+#  define _LIBCPP_ALWAYS_INLINE __forceinline
+#  define _LIBCPP_HAS_NO_VECTOR_EXTENSION
+#  define _LIBCPP_DISABLE_EXTENSION_WARNING
 
 #elif defined(_LIBCPP_COMPILER_IBM)
 
-#define _ALIGNAS(x) __attribute__((__aligned__(x)))
-#define _ALIGNAS_TYPE(x) __attribute__((__aligned__(_LIBCPP_ALIGNOF(x))))
-#define _ATTRIBUTE(x) __attribute__((x))
-#define _LIBCPP_NORETURN __attribute__((noreturn))
-
-#define _LIBCPP_HAS_NO_UNICODE_CHARS
-
-#if defined(_AIX)
-#define __MULTILOCALE_API
-#endif
-
-#define _LIBCPP_HAS_NO_ASAN
-
-#define _LIBCPP_ALWAYS_INLINE __attribute__ ((__always_inline__))
+#  if defined(_AIX)
+#    define __MULTILOCALE_API
+#  endif
 
-#define _LIBCPP_HAS_NO_VECTOR_EXTENSION
+#  define _LIBCPP_HAS_NO_ASAN
+#  define _LIBCPP_ALWAYS_INLINE __attribute__ ((__always_inline__))
+#  define _LIBCPP_HAS_NO_VECTOR_EXTENSION
+#  define _LIBCPP_DISABLE_EXTENSION_WARNING
 
-#define _LIBCPP_DISABLE_EXTENSION_WARNING
+typedef unsigned short char16_t;
+typedef unsigned int   char32_t;
 
 #endif // _LIBCPP_COMPILER_[CLANG|GCC|MSVC|IBM]
 
@@ -733,19 +699,6 @@
 #   define _LIBCPP_PREFERRED_OVERLOAD __attribute__ ((__enable_if__(true, "")))
 #endif
 
-#ifndef _LIBCPP_HAS_NO_NOEXCEPT
-#  define _NOEXCEPT noexcept
-#  define _NOEXCEPT_(x) noexcept(x)
-#else
-#  define _NOEXCEPT throw()
-#  define _NOEXCEPT_(x)
-#endif
-
-#ifdef _LIBCPP_HAS_NO_UNICODE_CHARS
-typedef unsigned short char16_t;
-typedef unsigned int   char32_t;
-#endif
-
 #ifndef __SIZEOF_INT128__
 #define _LIBCPP_HAS_NO_INT128
 #endif
@@ -950,12 +903,7 @@
   const void *, const void *, const void *, const void *);
 #endif
 
-// Try to find out if RTTI is disabled.
-#if defined(_LIBCPP_COMPILER_CLANG_BASED) && !__has_feature(cxx_rtti)
-#  define _LIBCPP_NO_RTTI
-#elif defined(__GNUC__) && !defined(__GXX_RTTI)
-#  define _LIBCPP_NO_RTTI
-#elif defined(_LIBCPP_COMPILER_MSVC) && !defined(_CPPRTTI)
+#if !defined(__cpp_rtti) || __cpp_rtti < 199711L
 #  define _LIBCPP_NO_RTTI
 #endif
 
@@ -1122,10 +1070,8 @@
 #endif
 
 // Use a function like macro to imply that it must be followed by a semicolon
-#if _LIBCPP_STD_VER > 14 && __has_cpp_attribute(fallthrough)
+#if __has_cpp_attribute(fallthrough)
 #  define _LIBCPP_FALLTHROUGH() [[fallthrough]]
-#elif __has_cpp_attribute(clang::fallthrough)
-#  define _LIBCPP_FALLTHROUGH() [[clang::fallthrough]]
 #elif __has_attribute(__fallthrough__)
 #  define _LIBCPP_FALLTHROUGH() __attribute__((__fallthrough__))
 #else
@@ -1218,18 +1164,12 @@
 #endif
 
 #if __has_attribute(init_priority)
-    // TODO: Remove this once we drop support for building libc++ with old Clangs
-#   if (defined(_LIBCPP_CLANG_VER) && _LIBCPP_CLANG_VER < 1200) || \
-       (defined(__apple_build_version__) && __apple_build_version__ < 13000000)
-#     define _LIBCPP_INIT_PRIORITY_MAX __attribute__((init_priority(101)))
-#   else
-#     define _LIBCPP_INIT_PRIORITY_MAX __attribute__((init_priority(100)))
-#   endif
+#  define _LIBCPP_INIT_PRIORITY_MAX __attribute__((init_priority(100)))
 #else
-# define _LIBCPP_INIT_PRIORITY_MAX
+#  define _LIBCPP_INIT_PRIORITY_MAX
 #endif
 
-#if defined(__GNUC__) || defined(__clang__)
+#if defined(_LIBCPP_COMPILER_CLANG_BASED) || defined(_LIBCPP_COMPILER_GCC)
   // The attribute uses 1-based indices for ordinary and static member functions.
   // The attribute uses 2-based indices for non-static member functions.
 # define _LIBCPP_ATTRIBUTE_FORMAT(archetype, format_string_index, first_format_arg_index) \