From 09f8f22a6451a4d88115cde4d6f19ffd1855a5d5 Mon Sep 17 00:00:00 2001
From: Walter Lee <49250218+googlewalt@users.noreply.github.com>
Date: Fri, 30 Jan 2026 10:01:51 -0500
Subject: [PATCH] [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.
---
 libcxx/include/stdatomic.h                    |  2 +-
 .../include_stdatomic_as_c.sh.cpp             | 30 +++++++++++++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)
 create mode 100644 libcxx/test/extensions/libcxx/depr/depr.c.headers/include_stdatomic_as_c.sh.cpp

diff --git a/libcxx/include/stdatomic.h b/libcxx/include/stdatomic.h
index 2991030eee45..e7b787560ddc 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 000000000000..31ad5c905367
--- /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;
+}