[clang] [clang] Define ATOMIC_FLAG_INIT correctly for C++. (PR #97534)

[Chris Copeland via cfe-commits]

https://github.com/chrisnc updated https://github.com/llvm/llvm-project/pull/97534

>From 1b39c59e0ec3cbe1f2cabc650b983883439dde31 Mon Sep 17 00:00:00 2001
From: Chris Copeland <chris at chrisnc.net>
Date: Wed, 3 Jul 2024 00:11:39 -0700
Subject: [PATCH] [clang] Define `ATOMIC_FLAG_INIT` correctly for C++.

---
 clang/docs/ReleaseNotes.rst    | 3 +++
 clang/lib/Headers/stdatomic.h  | 4 ++++
 clang/test/Headers/stdatomic.c | 5 +++++
 3 files changed, 12 insertions(+)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index d03459f7cc42c..587921850798b 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -891,6 +891,9 @@ Bug Fixes in This Version
 - Fixed an assertion failure when a template non-type parameter contains
   an invalid expression.
 
+- Fixed the definition of ``ATOMIC_FLAG_INIT`` in ``<stdatomic.h>`` so it can
+  be used in C++.
+
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
diff --git a/clang/lib/Headers/stdatomic.h b/clang/lib/Headers/stdatomic.h
index 2027055f38796..1991351f9e9ef 100644
--- a/clang/lib/Headers/stdatomic.h
+++ b/clang/lib/Headers/stdatomic.h
@@ -172,7 +172,11 @@ typedef _Atomic(uintmax_t)          atomic_uintmax_t;
 
 typedef struct atomic_flag { atomic_bool _Value; } atomic_flag;
 
+#ifdef __cplusplus
+#define ATOMIC_FLAG_INIT {false}
+#else
 #define ATOMIC_FLAG_INIT { 0 }
+#endif
 
 /* These should be provided by the libc implementation. */
 #ifdef __cplusplus
diff --git a/clang/test/Headers/stdatomic.c b/clang/test/Headers/stdatomic.c
index 3643fd4245b31..9afd531a9ed9b 100644
--- a/clang/test/Headers/stdatomic.c
+++ b/clang/test/Headers/stdatomic.c
@@ -1,5 +1,8 @@
 // RUN: %clang_cc1 -std=c11 -E %s | FileCheck %s
 // RUN: %clang_cc1 -std=c11 -fms-compatibility -E %s | FileCheck %s
+// RUN: %clang_cc1 -std=c11 %s -verify
+// RUN: %clang_cc1 -x c++ -std=c++11 %s -verify
+// expected-no-diagnostics
 #include <stdatomic.h>
 
 int bool_lock_free = ATOMIC_BOOL_LOCK_FREE;
@@ -31,3 +34,5 @@ int llong_lock_free = ATOMIC_LLONG_LOCK_FREE;
 
 int pointer_lock_free = ATOMIC_POINTER_LOCK_FREE;
 // CHECK: pointer_lock_free = {{ *[012] *;}}
+
+atomic_flag f = ATOMIC_FLAG_INIT;