[libc++] Allow the use of extensions in the implementation (#79532)

We've talked about allowing extensions on
[discourse](https://discourse.llvm.org/t/rfc-use-language-extensions-from-future-standards-in-libc/71898/5)
and in a libc++ monthly meeting and agreed to test it out in the LLVM 18
release. We've done that with the `tuple` constructor overload set
(using conditional `explicit`). Since we haven't heard about any
breakages, it seems safe to do. This patch enables the use of extension
from later C++ standards inside the versioned `std` namespaces. This
should be good enough, since almost all of our code is inside that
namespace. This approach also avoids the use of extensions inside the
test `std` suite. That part of the code base should stay clean, since
it's a test suite that is also used by other vendors to test their
implementations.

Author: philnik777

libcxx/include/__config

@@ -406,6 +406,10 @@ _LIBCPP_HARDENING_MODE_DEBUG
 #    define __has_include(...) 0
 #  endif
 
+#  ifndef __has_warning
+#    define __has_warning(...) 0
+#  endif
+
 #  if !defined(_LIBCPP_COMPILER_CLANG_BASED) && __cplusplus < 201103L
 #    error "libc++ only supports C++03 with Clang-based compilers. Please enable C++11"
 #  endif
@@ -723,6 +727,23 @@ typedef __char32_t char32_t;
 #    define _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION _LIBCPP_ALWAYS_INLINE
 #  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 _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_FAST
 #    define _LIBCPP_HARDENING_SIG f
 #  elif _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_EXTENSIVE
@@ -810,16 +831,33 @@ typedef __char32_t char32_t;
 #    define _LIBCPP_HIDE_FROM_ABI_AFTER_V1 _LIBCPP_HIDE_FROM_ABI
 #  endif
 
+// TODO: Remove this workaround once we drop support for Clang 16
+#if __has_warning("-Wc++23-extensions")
+#  define _LIBCPP_CLANG_DIAGNOSTIC_IGNORED_CXX23_EXTENSION _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wc++23-extensions")
+#else
+#  define _LIBCPP_CLANG_DIAGNOSTIC_IGNORED_CXX23_EXTENSION _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wc++2b-extensions")
+#endif
+
 // Inline namespaces are available in Clang/GCC/MSVC regardless of C++ dialect.
 // clang-format off
-#  define _LIBCPP_BEGIN_NAMESPACE_STD namespace _LIBCPP_TYPE_VISIBILITY_DEFAULT std {                                  \
+#  define _LIBCPP_BEGIN_NAMESPACE_STD _LIBCPP_DIAGNOSTIC_PUSH                                                          \
+                                      _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wc++11-extensions")                           \
+                                      _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_CXX23_EXTENSION                                 \
+                                      _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++23-extensions")                             \
+                                      namespace _LIBCPP_TYPE_VISIBILITY_DEFAULT std {                                  \
                                inline namespace _LIBCPP_ABI_NAMESPACE {
-#  define _LIBCPP_END_NAMESPACE_STD }}
+#  define _LIBCPP_END_NAMESPACE_STD }} _LIBCPP_DIAGNOSTIC_POP
 
 #  define _LIBCPP_BEGIN_NAMESPACE_FILESYSTEM _LIBCPP_BEGIN_NAMESPACE_STD                                               \
                                              inline namespace __fs { namespace filesystem {
 
-#  define _LIBCPP_END_NAMESPACE_FILESYSTEM _LIBCPP_END_NAMESPACE_STD }}
+#  define _LIBCPP_END_NAMESPACE_FILESYSTEM }} _LIBCPP_END_NAMESPACE_STD
 // clang-format on
 
 #  if __has_attribute(__enable_if__)
@@ -1256,23 +1294,6 @@ __sanitizer_verify_double_ended_contiguous_container(const void*, const void*, c
 // 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
-
 // c8rtomb() and mbrtoc8() were added in C++20 and C23. Support for these
 // functions is gradually being added to existing C libraries. The conditions
 // below check for known C library versions and conditions under which these

libcxx/include/tuple

@@ -548,10 +548,6 @@ class _LIBCPP_TEMPLATE_VIS tuple {
 public:
   // [tuple.cnstr]
 
-  _LIBCPP_DIAGNOSTIC_PUSH
-  _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wc++20-extensions")
-  _LIBCPP_GCC_DIAGNOSTIC_IGNORED("-Wc++20-extensions")
-
   // tuple() constructors (including allocator_arg_t variants)
   template <template <class...> class _IsImpDefault                = __is_implicitly_default_constructible,
             template <class...> class _IsDefault                   = is_default_constructible,
@@ -833,8 +829,6 @@ public:
       : __base_(allocator_arg_t(), __alloc, std::move(__p)) {}
 #  endif // _LIBCPP_STD_VER >= 23
 
-  _LIBCPP_DIAGNOSTIC_POP
-
   // [tuple.assign]
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 tuple&
   operator=(_If<_And<is_copy_assignable<_Tp>...>::value, tuple, __nat> const& __tuple)