Clang 14's `<stdatomic.h>` is incompatible with MSVC's `<stdatomic.h>` :radioactive:

[StephanTLavavej]

This repros with Clang 14, when the STL's tests are compiled with the official toolset (instead of the microsoft/STL repo - the difference is in the header locations, because Clang overrides toolset headers).

tests/std/tests/P0943R6_stdatomic_h/test.compile.pass.cpp and the many tests including stl/inc/__msvc_all_public_headers.hpp fail due to Clang's <stdatomic.h>, like this:

RUN.PM line 750:  Executing: "clang-cl -m32  /nologo /Od /W4 /w14061 /w14242 /w14265 /w14582 /w14583 /w14587 /w14588 /w14749 /w14841 /w14842 /w15038 /w15214 /w15215 /w15216 /w15217 /sdl /WX /Zc:strictStrings /D_ENABLE_STL_INTERNAL_CHECK /bigobj /FIforce_include.hpp /w14365 /D_ENFORCE_FACET_SPECIALIZATIONS=1 /D_STL_CALL_ABORT_INSTEAD_OF_INVALID_PARAMETER -fno-ms-compatibility -fno-delayed-template-parsing /EHsc /MD /std:c++14 /w14640 /Zc:threadSafeInit-   .\test.cpp  /c".
In file included from .\test.cpp:4:
In file included from D:\Contest\ca1z2udg.zif\binaries\x86chk\inc\__msvc_all_public_headers.hpp:146:
D:\Contest\ca1z2udg.zif\src\tools\LLVM\LLVM14\x86\lib\clang\14.0.5\include\stdatomic.h(70,6): error: conflicting types for 'atomic_thread_fence'
void atomic_thread_fence(memory_order);
     ^
D:\Contest\ca1z2udg.zif\binaries\x86chk\inc\atomic(166,24): note: previous definition is here
extern "C" inline void atomic_thread_fence(const memory_order _Order) noexcept {
                       ^
[...]

As a workaround, we're going to avoid including <stdatomic.h> for Clang, but solving the root cause would be better.

[fsb4000]

They have many macros:

#define atomic_thread_fence(order) __c11_atomic_thread_fence(order)
#define atomic_fetch_xor_explicit __c11_atomic_fetch_xor
....

and we have real functions instead of macros in <atomic>:

extern "C" inline void atomic_thread_fence(const memory_order _Order) noexcept {
    ::_Atomic_thread_fence(static_cast<unsigned int>(_Order));
}
....

and then macro replaces to

extern "C" inline void __c11_atomic_thread_fence(const memory_order _Order) noexcept {
    ::_Atomic_thread_fence(static_cast<unsigned int>(_Order));
}

but __c11_atomic_thread_fence is builtin.
So I think Clang just should include <stdatomic.h> if it is C++ and MSVC and use their <stdatomic.h> only for MSVC and C:
https://github.com/llvm/llvm-project/blob/6aff1b7b3ca71e048375a85a11e611527da6a45c/clang/lib/Headers/stdatomic.h#L13-L22

I prepared a patch: https://reviews.llvm.org/D130419

[fsb4000]

They landed the patch : llvm/llvm-project@ba49d39
Thanks Mark de Wever (@mordante) and Aaron Ballman (@AaronBallman) ❤️

[h-vetinari]

Speaking of <stdatomic.h> - the conformance page says visual studio does not support it, and I cannot find a corresponding issue in the dev community for it - is there a plan to bring this to conformance eventually?

[fsb4000]

Yes, DevCom-1204057

[h-vetinari]

Yes, DevCom-1204057

Thanks! Guess I was searching wrong (under "visual studio" instead of under "C++").

[mordante]

They landed the patch : llvm/llvm-project@ba49d39 Thanks Mark de Wever (@mordante) and Aaron Ballman (@AaronBallman) heart

You're welcome. FYI the patch landed before LLVM 15 was branched. So the fix will be in Clang-15.

[fsb4000]

__cplusplus and _STL_LANG are 202002L for clang-cl and /std:c++latest.
This is why the test is not enabled:

STL/tests/std/tests/P0943R6_stdatomic_h/test.compile.pass.cpp

Line 4 in c873cf0

#ifndef __clang__ // TRANSITION, GH-2862

The test works if I define the macro manually