[libc++] Don't implement <stdatomic.h> before C++23 (#123130)

https://github.com/llvm/llvm-project/pull/95498 implemented a libc++
extension where <stdatomic.h> would forward to <atomic> even before
C++23. Unfortunately, this was found to be a breaking change (with
fairly widespread impact) since that changes whether _Atomic(T) is a C
style atomic or std::atomic<T>. In principle, this can even be an ABI
break.

We generally don't implement extensions in libc++ because they cause so
many problems, and that extension had been accepted because it was
deemed pretty small and only a quality of life improvement. Since it has
widespread impact on valid C++20 (and before) code, this patch removes
the extension before we ship it in any public release.

libcxx/include/atomic

@@ -592,6 +592,16 @@ template <class T>
 #else
 #  include <__config>
 
+#  if defined(_LIBCPP_STDATOMIC_H) || defined(kill_dependency) || defined(atomic_load)
+#    define _LIBCPP_STDATOMIC_H_HAS_DEFINITELY_BEEN_INCLUDED 1
+#  else
+#    define _LIBCPP_STDATOMIC_H_HAS_DEFINITELY_BEEN_INCLUDED 0
+#  endif
+
+#  if _LIBCPP_STD_VER < 23 && _LIBCPP_STDATOMIC_H_HAS_DEFINITELY_BEEN_INCLUDED
+#    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_flag.h>

libcxx/include/stdatomic.h

@@ -126,7 +126,7 @@ using std::atomic_signal_fence                         // see below
 #    pragma GCC system_header
 #  endif
 
-#  if defined(__cplusplus)
+#  if defined(__cplusplus) && _LIBCPP_STD_VER >= 23
 
 #    include <atomic>
 #    include <version>
@@ -231,13 +231,17 @@ 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;
 
-#  else
+#  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.
 #    if __has_include_next(<stdatomic.h>)
 #      include_next <stdatomic.h>
 #    endif
 
-#  endif // defined(__cplusplus)
+#  endif // defined(__cplusplus) && _LIBCPP_STD_VER >= 23
 #endif   // defined(__cplusplus) && __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
 
 #endif // _LIBCPP_STDATOMIC_H

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

@@ -7,15 +7,16 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: no-threads
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
 
 // XFAIL: FROZEN-CXX03-HEADERS-FIXME
 
-// This test verifies that <stdatomic.h> redirects to <atomic>. As an extension,
-// libc++ enables this redirection even before C++23.
+// This test verifies that <stdatomic.h> redirects to <atomic>.
 
-// 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.
+// Before C++23, <stdatomic.h> can be included after <atomic>, but including it
+// first doesn't work because its macros break <atomic>. Fixing that is the point
+// of the C++23 change that added <stdatomic.h> to C++. Thus, this test verifies
+// that <stdatomic.h> can be included first.
 #include <stdatomic.h>
 #include <atomic>
 

libcxx/test/libcxx/atomics/atomics.syn/incompatible_with_stdatomic.verify.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
+// REQUIRES: c++03 || c++11 || c++14 || c++17 || c++20
+
+// 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/dont_hijack_header.compile.pass.cpp

@@ -0,0 +1,24 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 ensures that we don't hijack the <stdatomic.h> header (e.g. by providing
+// an empty header) even when compiling before C++23, since some users were using the
+// Clang or platform provided header before libc++ added its own.
+
+// On GCC, the compiler-provided <stdatomic.h> is not C++ friendly, so including <stdatomic.h>
+// doesn't work at all if we don't use the <stdatomic.h> provided by libc++ in C++23 and above.
+// XFAIL: (c++11 || c++14 || c++17 || c++20) && gcc
+
+#include <stdatomic.h>
+
+void f() {
+  atomic_int i; // just make sure the header isn't empty
+  (void)i;
+}

libcxx/test/libcxx/atomics/stdatomic.h.syn/dont_hijack_header.cxx23.compile.pass.cpp

@@ -0,0 +1,28 @@
+//===----------------------------------------------------------------------===//
+//
+// 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> DOES NOT redirect to <atomic> before C++23,
+// since doing so is a breaking change. Several things can break when that happens,
+// because the type of _Atomic(T) changes from _Atomic(T) to std::atomic<T>.
+//
+// For example, redeclarations can become invalid depending on whether they
+// have been declared with <stdatomic.h> in scope or not.
+
+// REQUIRES: c++03 || c++11 || c++14 || c++17 || c++20
+
+// On GCC, the compiler-provided <stdatomic.h> is not C++ friendly, so including <stdatomic.h>
+// doesn't work at all if we don't use the <stdatomic.h> provided by libc++ in C++23 and above.
+// XFAIL: (c++11 || c++14 || c++17 || c++20) && gcc
+
+#include <atomic>
+#include <stdatomic.h>
+#include <type_traits>
+
+static_assert(!std::is_same<_Atomic(int), std::atomic<int> >::value, "");