[libc++] Enable C++ stdatomic.h for all C++ versions (llvm#95498)

This extension is motivated by Android's use of libc++, where
<stdatomic.h> has redirected to <atomic> for many years, long before it
was standardized in C++23.

When libc++'s stdatomic.h is included in C translation units, delegate
to the next stdatomic.h, which could come from Clang or libc.

Author: rprichard

libcxx/include/atomic

@@ -589,10 +589,6 @@ template <class T>
 
 #include <__config>
 
-#if _LIBCPP_STD_VER < 23 && defined(_LIBCPP_STDATOMIC_H)
-#  error <atomic> is incompatible with <stdatomic.h> before C++23. Please compile with -std=c++23.
-#endif
-
 #include <__atomic/aliases.h>
 #include <__atomic/atomic.h>
 #include <__atomic/atomic_base.h>

libcxx/include/stdatomic.h

@@ -123,7 +123,7 @@ using std::atomic_signal_fence                         // see below
 #  pragma GCC system_header
 #endif
 
-#if defined(__cplusplus) && _LIBCPP_STD_VER >= 23
+#if defined(__cplusplus)
 
 #  include <atomic>
 #  include <version>
@@ -156,10 +156,14 @@ using std::atomic_long _LIBCPP_USING_IF_EXISTS;
 using std::atomic_ulong _LIBCPP_USING_IF_EXISTS;
 using std::atomic_llong _LIBCPP_USING_IF_EXISTS;
 using std::atomic_ullong _LIBCPP_USING_IF_EXISTS;
+#  ifndef _LIBCPP_HAS_NO_CHAR8_T
 using std::atomic_char8_t _LIBCPP_USING_IF_EXISTS;
+#  endif
 using std::atomic_char16_t _LIBCPP_USING_IF_EXISTS;
 using std::atomic_char32_t _LIBCPP_USING_IF_EXISTS;
+#  ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 using std::atomic_wchar_t _LIBCPP_USING_IF_EXISTS;
+#  endif
 
 using std::atomic_int8_t _LIBCPP_USING_IF_EXISTS;
 using std::atomic_uint8_t _LIBCPP_USING_IF_EXISTS;
@@ -224,16 +228,12 @@ using std::atomic_store_explicit _LIBCPP_USING_IF_EXISTS;
 using std::atomic_signal_fence _LIBCPP_USING_IF_EXISTS;
 using std::atomic_thread_fence _LIBCPP_USING_IF_EXISTS;
 
-#elif defined(_LIBCPP_COMPILER_CLANG_BASED)
+#else
 
-// Before C++23, we include the next <stdatomic.h> on the path to avoid hijacking
-// the header. We do this because Clang has historically shipped a <stdatomic.h>
-// header that would be available in all Standard modes, and we don't want to
-// break that use case.
 #  if __has_include_next(<stdatomic.h>)
 #    include_next <stdatomic.h>
 #  endif
 
-#endif // defined(__cplusplus) && _LIBCPP_STD_VER >= 23
+#endif // defined(__cplusplus)
 
 #endif // _LIBCPP_STDATOMIC_H

libcxx/test/libcxx/atomics/atomics.syn/compatible_with_stdatomic.compile.pass.cpp

@@ -0,0 +1,22 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: no-threads
+
+// This test verifies that <stdatomic.h> redirects to <atomic>. As an extension,
+// libc++ enables this redirection even before C++23.
+
+// Ordinarily, <stdatomic.h> can be included after <atomic>, but including it
+// first doesn't work because its macros break <atomic>. Verify that
+// <stdatomic.h> can be included first.
+#include <stdatomic.h>
+#include <atomic>
+
+#include <type_traits>
+
+static_assert(std::is_same<atomic_int, std::atomic<int> >::value, "");