[libcxx-commits] [libcxx] 09f8f22 - [libc++] "Always" include_next for non C++ path in stdatomic.h (#178463)

[via libcxx-commits]

Author: Walter Lee
Date: 2026-01-30T10:01:51-05:00
New Revision: 09f8f22a6451a4d88115cde4d6f19ffd1855a5d5

URL: https://github.com/llvm/llvm-project/commit/09f8f22a6451a4d88115cde4d6f19ffd1855a5d5
DIFF: https://github.com/llvm/llvm-project/commit/09f8f22a6451a4d88115cde4d6f19ffd1855a5d5.diff

LOG: [libc++] "Always" include_next for non C++ path in stdatomic.h (#178463)

In https://github.com/llvm/llvm-project/pull/176903, `#include
<__configuration/compiler.h>` is moved into the
`#ifdef _cplusplus` clause, so `_LIBCPP_COMPILER_CLANG_BASED` is no
longer set for C compiles. This causes a regression internally, where
when C compiles includes stdatomic.h, they no longer get the
corresponding C header.

C++ stdlib headers "shouldn't" be on the search patch for C compile, but
we do and so do lots of other people, so libc++ tends to support that.
This include_next for a C compile should be unconditional, not
conditional upon being Clang.

Added: 
    libcxx/test/extensions/libcxx/depr/depr.c.headers/include_stdatomic_as_c.sh.cpp

Modified: 
    libcxx/include/stdatomic.h

Removed: 
    


################################################################################
diff  --git a/libcxx/include/stdatomic.h b/libcxx/include/stdatomic.h
index 2991030eee456..e7b787560ddc4 100644
--- a/libcxx/include/stdatomic.h
+++ b/libcxx/include/stdatomic.h
@@ -231,7 +231,7 @@ 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)
+#  elif !defined(__cplusplus) || 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>

diff  --git a/libcxx/test/extensions/libcxx/depr/depr.c.headers/include_stdatomic_as_c.sh.cpp b/libcxx/test/extensions/libcxx/depr/depr.c.headers/include_stdatomic_as_c.sh.cpp
new file mode 100644
index 0000000000000..31ad5c9053675
--- /dev/null
+++ b/libcxx/test/extensions/libcxx/depr/depr.c.headers/include_stdatomic_as_c.sh.cpp
@@ -0,0 +1,30 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// We're building as C, so this test doesn't work when building with modules.
+// UNSUPPORTED: clang-modules-build
+
+// GCC complains about unrecognized arguments because we're compiling the
+// file as C, but we're passing C++ flags on the command-line.
+// UNSUPPORTED: gcc
+
+// Test that stdatomic.h gets the C header with its definitions.
+
+// NOTE: It's not common or recommended to have libc++ in the header search
+// path when compiling C files, but it does happen often enough.
+
+// RUN: %{cxx} -c -xc %s -fsyntax-only %{flags} %{compile_flags} -std=c99
+
+#include <stdatomic.h>
+
+int main(int argc, char** argv) {
+  (void)argc;
+  (void)argv;
+  [[maybe_unused]] atomic_bool x;
+  return 0;
+}