patches/Revert-libc-Don-t-implement-stdatomic.h-before-C-23-.patch

From f9a1f3b7e70d960338b255e376741417fc52cb92 Mon Sep 17 00:00:00 2001
From: Yi Kong <yikong@google.com>
Date: Wed, 22 Jan 2025 14:03:48 +0800
Subject: [PATCH] Revert "[libc++] Don't implement <stdatomic.h> before C++23
 (#123130)"

This reverts commit bbd871e2baad2e74dbde202823b3439d2a96d3f8.
---
 libcxx/include/atomic                         | 10 -------
 libcxx/include/stdatomic.h                    | 10 ++-----
 ...compatible_with_stdatomic.compile.pass.cpp | 11 ++++----
 .../incompatible_with_stdatomic.verify.cpp    | 22 ---------------
 .../dont_hijack_header.compile.pass.cpp       | 24 ----------------
 .../dont_hijack_header.cxx23.compile.pass.cpp | 28 -------------------
 6 files changed, 8 insertions(+), 97 deletions(-)
 delete mode 100644 libcxx/test/libcxx/atomics/atomics.syn/incompatible_with_stdatomic.verify.cpp
 delete mode 100644 libcxx/test/libcxx/atomics/stdatomic.h.syn/dont_hijack_header.compile.pass.cpp
 delete mode 100644 libcxx/test/libcxx/atomics/stdatomic.h.syn/dont_hijack_header.cxx23.compile.pass.cpp

diff --git a/libcxx/include/atomic b/libcxx/include/atomic
index 75af5de33ca4..80f9e437bfaa 100644
--- a/libcxx/include/atomic
+++ b/libcxx/include/atomic
@@ -592,16 +592,6 @@ 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>
diff --git a/libcxx/include/stdatomic.h b/libcxx/include/stdatomic.h
index 2991030eee45..a0b46e3b7bc1 100644
--- a/libcxx/include/stdatomic.h
+++ b/libcxx/include/stdatomic.h
@@ -126,7 +126,7 @@ 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>
@@ -231,17 +231,13 @@ 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   // defined(__cplusplus) && __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
 
 #endif // _LIBCPP_STDATOMIC_H
diff --git a/libcxx/test/libcxx/atomics/atomics.syn/compatible_with_stdatomic.compile.pass.cpp b/libcxx/test/libcxx/atomics/atomics.syn/compatible_with_stdatomic.compile.pass.cpp
index 30e9672a2568..323072da1446 100644
--- a/libcxx/test/libcxx/atomics/atomics.syn/compatible_with_stdatomic.compile.pass.cpp
+++ b/libcxx/test/libcxx/atomics/atomics.syn/compatible_with_stdatomic.compile.pass.cpp
@@ -7,16 +7,15 @@
 //===----------------------------------------------------------------------===//
 
 // 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>.
+// This test verifies that <stdatomic.h> redirects to <atomic>. As an extension,
+// libc++ enables this redirection even before C++23.
 
-// 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.
+// 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>
 
diff --git a/libcxx/test/libcxx/atomics/atomics.syn/incompatible_with_stdatomic.verify.cpp b/libcxx/test/libcxx/atomics/atomics.syn/incompatible_with_stdatomic.verify.cpp
deleted file mode 100644
index ca092d9c6027..000000000000
--- a/libcxx/test/libcxx/atomics/atomics.syn/incompatible_with_stdatomic.verify.cpp
+++ /dev/null
@@ -1,22 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// 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.}}
diff --git a/libcxx/test/libcxx/atomics/stdatomic.h.syn/dont_hijack_header.compile.pass.cpp b/libcxx/test/libcxx/atomics/stdatomic.h.syn/dont_hijack_header.compile.pass.cpp
deleted file mode 100644
index 6df80daf9414..000000000000
--- a/libcxx/test/libcxx/atomics/stdatomic.h.syn/dont_hijack_header.compile.pass.cpp
+++ /dev/null
@@ -1,24 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// 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;
-}
diff --git a/libcxx/test/libcxx/atomics/stdatomic.h.syn/dont_hijack_header.cxx23.compile.pass.cpp b/libcxx/test/libcxx/atomics/stdatomic.h.syn/dont_hijack_header.cxx23.compile.pass.cpp
deleted file mode 100644
index a8a99e6937f3..000000000000
--- a/libcxx/test/libcxx/atomics/stdatomic.h.syn/dont_hijack_header.cxx23.compile.pass.cpp
+++ /dev/null
@@ -1,28 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// 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, "");
-- 
2.48.0.rc2.279.g1de40edade-goog