[libc++] Allow the use of extensions from later standards in namespace std

[philnik]

Summary

This allows us to simplify some code drastically and improve our QoI.

[ldionne]

Seeing how much this simplifies D148432, I must say I kind of want to just do it.

However, this places an incredibly strict requirement on the library that all supported compilers are tested in the CI. Otherwise, we'll quickly start breaking the library with some compilers if they don't support those extensions. Similarly, this could make it more difficult to port the library to new compilers. We don't do that often and we could also require that new compiler ports only be for the most recent standards version (for example we'd support e.g. MSVC but only in C++23 mode).

All in all, I don't think the above is a dealbreaker. However, I would argue in favour of a slightly more gradual approach just to get a bit of experience with this before going completely crazy on it. For example, could we enable something like *just* the C++20 extensions for the conditional explicit feature within tuple (and nowhere else for now). And then after one release, we'd probably realize that it's no big deal, nobody noticed and it doesn't break anything, and we could do the whole thing. What I'm scared of doing is to turn on the whole thing, start doing refactorings and then realize that we have a big big problem that we can very hardly just git revert out of.

So yeah, I am in favour but I would thread carefully and start with changes in a focused area for one release, and see what we learn if anything. If we don't learn anything, then this is truly NFC and let's go crazy on it -- we don't lose anything and we gain a bunch.

libcxx/include/__config

 #    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
 
+#  ifdef _LIBCPP_COMPILER_CLANG_BASED
+#    define _LIBCPP_DIAGNOSTIC_PUSH _Pragma("clang diagnostic push")
+#    define _LIBCPP_DIAGNOSTIC_POP _Pragma("clang diagnostic pop")
+#    define _LIBCPP_CLANG_DIAGNOSTIC_IGNORED(str) _Pragma(_LIBCPP_TOSTRING(clang diagnostic ignored str))
+#    define _LIBCPP_GCC_DIAGNOSTIC_IGNORED(str)
+#  elif defined(_LIBCPP_COMPILER_GCC)
+#    define _LIBCPP_DIAGNOSTIC_PUSH _Pragma("GCC diagnostic push")
+#    define _LIBCPP_DIAGNOSTIC_POP _Pragma("GCC diagnostic pop")
+#    define _LIBCPP_CLANG_DIAGNOSTIC_IGNORED(str)
+#    define _LIBCPP_GCC_DIAGNOSTIC_IGNORED(str) _Pragma(_LIBCPP_TOSTRING(GCC diagnostic ignored str))
+#  else
+#    define _LIBCPP_DIAGNOSTIC_PUSH
+#    define _LIBCPP_DIAGNOSTIC_POP
+#    define _LIBCPP_CLANG_DIAGNOSTIC_IGNORED(str)
+#    define _LIBCPP_GCC_DIAGNOSTIC_IGNORED(str)
+#  endif
+
+#  if defined(_AIX) && !defined(_LIBCPP_COMPILER_GCC)
+#    define _LIBCPP_PACKED_BYTE_FOR_AIX _Pragma("pack(1)")
+#    define _LIBCPP_PACKED_BYTE_FOR_AIX_END _Pragma("pack(pop)")
+#  else
+#    define _LIBCPP_PACKED_BYTE_FOR_AIX     /* empty */
+#    define _LIBCPP_PACKED_BYTE_FOR_AIX_END /* empty */
+#  endif
+
 // Inline namespaces are available in Clang/GCC/MSVC regardless of C++ dialect.
 // clang-format off
-#  define _LIBCPP_BEGIN_NAMESPACE_STD namespace std { inline namespace _LIBCPP_ABI_NAMESPACE {
-#  define _LIBCPP_END_NAMESPACE_STD }}
+#  define _LIBCPP_BEGIN_NAMESPACE_STD _LIBCPP_DIAGNOSTIC_PUSH                                                          \
+                                      _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wc++14-extensions")                           \
+                                      _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wc++17-extensions")                           \
+                                      _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wc++20-extensions")                           \
+                                      _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wc++2b-extensions")                           \
+                                      _LIBCPP_GCC_DIAGNOSTIC_IGNORED("-Wc++14-extensions")                             \
+                                      _LIBCPP_GCC_DIAGNOSTIC_IGNORED("-Wc++17-extensions")                             \
+                                      _LIBCPP_GCC_DIAGNOSTIC_IGNORED("-Wc++20-extensions")                             \
+                                      _LIBCPP_GCC_DIAGNOSTIC_IGNORED("-Wc++2b-extensions")                             \
+                                      namespace std { inline namespace _LIBCPP_ABI_NAMESPACE {
+
+#  define _LIBCPP_END_NAMESPACE_STD }} _LIBCPP_DIAGNOSTIC_POP
 #  define _VSTD std
 
 _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

 // Note that this can be replaced by #error as soon as clang-cl
 // implements msvc::no_unique_address, since there should be no C++20
 // compiler that doesn't support one of the two attributes at that point.
 // We generally don't want to use this macro outside of C++20-only code,
 // because using it conditionally in one language version only would make
 // the ABI inconsistent.
 #  endif
 
-#  ifdef _LIBCPP_COMPILER_CLANG_BASED
-#    define _LIBCPP_DIAGNOSTIC_PUSH _Pragma("clang diagnostic push")
-#    define _LIBCPP_DIAGNOSTIC_POP _Pragma("clang diagnostic pop")
-#    define _LIBCPP_CLANG_DIAGNOSTIC_IGNORED(str) _Pragma(_LIBCPP_TOSTRING(clang diagnostic ignored str))
-#    define _LIBCPP_GCC_DIAGNOSTIC_IGNORED(str)
-#  elif defined(_LIBCPP_COMPILER_GCC)
-#    define _LIBCPP_DIAGNOSTIC_PUSH _Pragma("GCC diagnostic push")
-#    define _LIBCPP_DIAGNOSTIC_POP _Pragma("GCC diagnostic pop")
-#    define _LIBCPP_CLANG_DIAGNOSTIC_IGNORED(str)
-#    define _LIBCPP_GCC_DIAGNOSTIC_IGNORED(str) _Pragma(_LIBCPP_TOSTRING(GCC diagnostic ignored str))
-#  else
-#    define _LIBCPP_DIAGNOSTIC_PUSH
-#    define _LIBCPP_DIAGNOSTIC_POP
-#    define _LIBCPP_CLANG_DIAGNOSTIC_IGNORED(str)
-#    define _LIBCPP_GCC_DIAGNOSTIC_IGNORED(str)
-#  endif
-
-#  if defined(_AIX) && !defined(_LIBCPP_COMPILER_GCC)
-#    define _LIBCPP_PACKED_BYTE_FOR_AIX _Pragma("pack(1)")
-#    define _LIBCPP_PACKED_BYTE_FOR_AIX_END _Pragma("pack(pop)")
-#  else
-#    define _LIBCPP_PACKED_BYTE_FOR_AIX     /* empty */
-#    define _LIBCPP_PACKED_BYTE_FOR_AIX_END /* empty */
-#  endif
-
 #  if __has_attribute(__packed__)
 #    define _LIBCPP_PACKED __attribute__((__packed__))
 #  else
 #    define _LIBCPP_PACKED
 #  endif
 
 // c8rtomb() and mbrtoc8() were added in C++20 and C23. Support for these
 // functions is gradually being added to existing C libraries. The conditions