[libc++] Remove workaround for C11 features on compilers that don't support using_if_exists

Instead of carrying around #ifdefs to determine whether those functions
are available on the platform, unconditionally use the using_if_exists
attribute to import it into namespace std only when available. That was
the purpose of this attribute from the start.

This change means that trying to use libc++ with an old SDK (or on an
old platform for platforms that ship system headers in /usr/include)
will require a recent Clang that supports the using_if_exists attribute.
When using an older Clang or GCC, the underlying platform has to support
a C11 standard library.

Differential Revision: https://reviews.llvm.org/D108203

Author: ldionne

libcxx/include/__config

@@ -294,9 +294,6 @@
 #    define _LIBCPP_HAS_BITSCAN64
 #  endif
 #  define _LIBCPP_HAS_OPEN_WITH_WCHAR
-#  if defined(_LIBCPP_MSVCRT)
-#    define _LIBCPP_HAS_QUICK_EXIT
-#  endif
 
 // Some CRT APIs are unavailable to store apps
 #  if defined(WINAPI_FAMILY)
@@ -392,72 +389,6 @@
 #  define _LIBCPP_NO_CFI
 #endif
 
-// If the compiler supports using_if_exists, pretend we have those functions and they'll
-// be picked up if the C library provides them.
-//
-// TODO: Once we drop support for Clang 12, we can assume the compiler supports using_if_exists
-//       for platforms that don't have a conforming C11 library, so we can drop this whole thing.
-#if __has_attribute(using_if_exists)
-# define _LIBCPP_HAS_TIMESPEC_GET
-# define _LIBCPP_HAS_QUICK_EXIT
-# define _LIBCPP_HAS_ALIGNED_ALLOC
-#else
-#if (defined(__ISO_C_VISIBLE) && (__ISO_C_VISIBLE >= 2011)) || __cplusplus >= 201103L
-#  if defined(__FreeBSD__)
-#    define _LIBCPP_HAS_ALIGNED_ALLOC
-#    define _LIBCPP_HAS_QUICK_EXIT
-#    if __FreeBSD_version >= 1300064 || \
-       (__FreeBSD_version >= 1201504 && __FreeBSD_version < 1300000)
-#      define _LIBCPP_HAS_TIMESPEC_GET
-#    endif
-#  elif defined(__BIONIC__)
-#    if __ANDROID_API__ >= 21
-#      define _LIBCPP_HAS_QUICK_EXIT
-#    endif
-#    if __ANDROID_API__ >= 28
-#      define _LIBCPP_HAS_ALIGNED_ALLOC
-#    endif
-#    if __ANDROID_API__ >= 29
-#      define _LIBCPP_HAS_TIMESPEC_GET
-#    endif
-#  elif defined(__Fuchsia__) || defined(__wasi__) || defined(__NetBSD__)
-#    define _LIBCPP_HAS_ALIGNED_ALLOC
-#    define _LIBCPP_HAS_QUICK_EXIT
-#    define _LIBCPP_HAS_TIMESPEC_GET
-#  elif defined(__OpenBSD__)
-#    define _LIBCPP_HAS_ALIGNED_ALLOC
-#    define _LIBCPP_HAS_TIMESPEC_GET
-#  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_ALIGNED_ALLOC
-#        define _LIBCPP_HAS_TIMESPEC_GET
-#      endif
-#    else // defined(_LIBCPP_HAS_MUSL_LIBC)
-#      define _LIBCPP_HAS_ALIGNED_ALLOC
-#      define _LIBCPP_HAS_QUICK_EXIT
-#      define _LIBCPP_HAS_TIMESPEC_GET
-#    endif
-#  elif defined(_LIBCPP_MSVCRT)
-     // Using Microsoft's C Runtime library, not MinGW
-#    define _LIBCPP_HAS_TIMESPEC_GET
-#  elif defined(__APPLE__)
-     // timespec_get and aligned_alloc were introduced in macOS 10.15 and
-     // aligned releases
-#    if ((defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ >= 101500) || \
-         (defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ >= 130000) || \
-         (defined(__ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__ >= 130000) || \
-         (defined(__ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__ >= 60000))
-#      define _LIBCPP_HAS_ALIGNED_ALLOC
-#      define _LIBCPP_HAS_TIMESPEC_GET
-#    endif
-#  endif // __APPLE__
-#endif
-#endif // __has_attribute(using_if_exists)
-
 #ifndef _LIBCPP_CXX03_LANG
 # define _LIBCPP_ALIGNOF(_Tp) alignof(_Tp)
 #elif defined(_LIBCPP_COMPILER_CLANG_BASED)

libcxx/include/cstdlib

@@ -132,11 +132,11 @@ using ::mbtowc _LIBCPP_USING_IF_EXISTS;
 using ::wctomb _LIBCPP_USING_IF_EXISTS;
 using ::mbstowcs _LIBCPP_USING_IF_EXISTS;
 using ::wcstombs _LIBCPP_USING_IF_EXISTS;
-#if !defined(_LIBCPP_CXX03_LANG) && defined(_LIBCPP_HAS_QUICK_EXIT)
+#if !defined(_LIBCPP_CXX03_LANG)
 using ::at_quick_exit _LIBCPP_USING_IF_EXISTS;
 using ::quick_exit _LIBCPP_USING_IF_EXISTS;
 #endif
-#if _LIBCPP_STD_VER > 14 && defined(_LIBCPP_HAS_ALIGNED_ALLOC)
+#if _LIBCPP_STD_VER > 14
 using ::aligned_alloc _LIBCPP_USING_IF_EXISTS;
 #endif
 

libcxx/include/ctime

@@ -52,27 +52,13 @@ int timespec_get( struct timespec *ts, int base); // C++17
 #  pragma GCC system_header
 #endif
 
-// FIXME:
-// Apple SDKs don't define ::timespec_get unconditionally in C++ mode. This
-// should be fixed in future SDKs, but for the time being we need to avoid
-// trying to use that declaration when the SDK doesn't provide it. Note that
-// we're detecting this here instead of in <__config> because we can't include
-// system headers from <__config>, since it leads to circular module dependencies.
-// This is also meant to be a very temporary workaround until the SDKs are fixed.
-#if defined(__APPLE__) && !__has_attribute(using_if_exists)
-#   include <sys/cdefs.h>
-#   if defined(_LIBCPP_HAS_TIMESPEC_GET) && (__DARWIN_C_LEVEL < __DARWIN_C_FULL)
-#       define _LIBCPP_HAS_TIMESPEC_GET_NOT_ACTUALLY_PROVIDED
-#   endif
-#endif
-
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 using ::clock_t _LIBCPP_USING_IF_EXISTS;
 using ::size_t _LIBCPP_USING_IF_EXISTS;
 using ::time_t _LIBCPP_USING_IF_EXISTS;
 using ::tm _LIBCPP_USING_IF_EXISTS;
-#if _LIBCPP_STD_VER > 14 && defined(_LIBCPP_HAS_TIMESPEC_GET)
+#if _LIBCPP_STD_VER > 14
 using ::timespec _LIBCPP_USING_IF_EXISTS;
 #endif
 using ::clock _LIBCPP_USING_IF_EXISTS;
@@ -84,7 +70,7 @@ using ::ctime _LIBCPP_USING_IF_EXISTS;
 using ::gmtime _LIBCPP_USING_IF_EXISTS;
 using ::localtime _LIBCPP_USING_IF_EXISTS;
 using ::strftime _LIBCPP_USING_IF_EXISTS;
-#if _LIBCPP_STD_VER > 14 && defined(_LIBCPP_HAS_TIMESPEC_GET) && !defined(_LIBCPP_HAS_TIMESPEC_GET_NOT_ACTUALLY_PROVIDED)
+#if _LIBCPP_STD_VER > 14
 using ::timespec_get _LIBCPP_USING_IF_EXISTS;
 #endif