[libc++] Remove overly conservative error in <stdatomic.h>

[ldionne]

Summary

As mentionned in D97044, it is fine if users include <atomic> and then
include <stdatomic.h> -- we don't need to error out for that case.

[ldionne]

@rnk @dblaikie Can you perhaps see if this patch helps with the issues you were encountering?

[rnk]

Sorry, I can't easily test this patch, but I do think it's probably the right direction.

[ldionne]

Fix CI issues

libcxx/include/stdatomic.h

 using std::atomic_thread_fence _LIBCPP_USING_IF_EXISTS;
 
 #elif defined(_LIBCPP_COMPILER_CLANG_BASED)
 
 // 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.
-//
-// However, if the user has already used <atomic> before, the two headers are
-// incompatible before C++23, so we issue a clear error here to avoid obscure
-// issues down the line.
 # if __has_include_next(<stdatomic.h>)
-#   ifdef _LIBCPP_ATOMIC
-#     error <stdatomic.h> is incompatible with <atomic> before C++23. Please compile with -std=c++23.
-#   endif
 #   include_next <stdatomic.h>
 # endif
 
 #endif // _LIBCPP_STD_VER > 20
 
 #endif // _LIBCPP_STDATOMIC_H

libcxx/test/libcxx/atomics/atomics.syn/incompatible_with_stdatomic.verify.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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: libcpp-has-no-threads
 // REQUIRES: c++03 || c++11 || c++14 || c++17 || c++20
 
-// This test ensures that we issue a reasonable diagnostic when using <atomic> while <stdatomic.h>
-// is in use too. Before C++23, this otherwise leads to obscure errors because <atomic> may try
-// to redefine things defined by <stdatomic.h>.
+// This test ensures that we issue a reasonable diagnostic when including <atomic> after
+// <stdatomic.h> has been included. Before C++23, this otherwise leads to obscure errors
+// because <atomic> may try to redefine things defined by <stdatomic.h>.
 
 // Ignore additional weird errors that happen when the two headers are mixed.
 // ADDITIONAL_COMPILE_FLAGS: -Xclang -verify-ignore-unexpected=error -Xclang -verify-ignore-unexpected=warning
 
 #include <stdatomic.h>
 #include <atomic>
 
 // expected-error@*:* {{<atomic> is incompatible with <stdatomic.h> before C++23.}}

libcxx/test/libcxx/atomics/stdatomic.h.syn/incompatible_with_atomic.verify.cpp

-//===----------------------------------------------------------------------===//
-//
-// 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: libcpp-has-no-threads
-// REQUIRES: c++03 || c++11 || c++14 || c++17 || c++20
-
-// This test ensures that we issue a reasonable diagnostic when using <stdatomic.h> while <atomic>
-// is in use too. Before C++23, this otherwise leads to obscure errors because <stdatomic.h> tries
-// to redefine things defined by <atomic>.
-
-// Ignore additional weird errors that happen when the two headers are mixed.
-// ADDITIONAL_COMPILE_FLAGS: -Xclang -verify-ignore-unexpected=error -Xclang -verify-ignore-unexpected=warning
-
-#include <atomic>
-#include <stdatomic.h>
-
-// expected-error@*:* {{<stdatomic.h> is incompatible with <atomic> before C++23}}