[libcxx-commits] [PATCH] D131435: [libcxx] Make stdatomic.h work when included from a C source file

[Shoaib Meenai via Phabricator via libcxx-commits]

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If a C source file includes the libc++ stdatomic.h, compilation will
break because (a) the C++ standard check will fail (which is expected),
and (b) `_LIBCPP_COMPILER_CLANG_BASED` won't be defined because the
logic defining it in `__config` is guarded by a `__cplusplus` check, so
we'll end up with a blank header. Check for `__clang__` directly, which
is equivalent, to make the second check pass even in a C context when
you're using Clang.

In an ideal world, a C source file wouldn't be including the libc++
header directory in its search path, so we'd never have this issue.
Unfortunately, certain build environments make this hard to guarantee,
and in this case it's easy to tweak this header to make it work in a C
context, so I'm hoping this is acceptable.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D131435

Files:
  libcxx/include/stdatomic.h


Index: libcxx/include/stdatomic.h
===================================================================
--- libcxx/include/stdatomic.h
+++ libcxx/include/stdatomic.h
@@ -220,7 +220,7 @@
 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(__clang__)
 
 // 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>


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