Headers: tweak inclusion condition for stdatomic.h

[compnerd]

Summary

MSVC requires that C++23 be available (_HAS_CXX23) else the entire
content is elided. Conditionalise the inclusion properly so that C/C++
code using stdatomic.h for memory_order_* constants are able to do
so without changing the C++ standard. This repairs builds of Swift and
libdispatch after ba49d39b20cc5358da28af2ac82bd336028780bc.

[Mordante]

Thanks for the patch!

clang/lib/Headers/stdatomic.h

21

Is __cplusplus-0 intentional? If so please add some comments how this differs from __cplusplus.
Since this is a C header we should test whether the macro exists before querying its value.

[compnerd]

clang/lib/Headers/stdatomic.h

21

Yes, that is intentional. The -0 is a trick! That avoids the need to check for the definition, because it will be evaluated to as the value of __cplusplus or be 0 due to the expansion to CPP rules and -0.

[compnerd]

Update to avoid use of the -0 trick.

[aaron.ballman]

Thanks for working on this! The changes LGTM. I've been trying to figure out if there's a way we can add test coverage for this change, but I'm really not coming up with much. However, the changes definitely should have a release note.

clang/lib/Headers/stdatomic.h

23

202002l -> 202002L to make it easier on people's eyeballs.

[Mordante]

LGTM after addressing @aaron.ballman's comments.

clang/lib/Headers/stdatomic.h

21

Ah yes of course. I really prefer the current version without the trick.

[thakis]

We think this is incorrect, see comments here: https://boringssl-review.googlesource.com/c/boringssl/+/55845/3/crypto/refcount_c11.c#18

[thakis]

(and https://bugs.chromium.org/p/chromium/issues/detail?id=1402885#c3)

[compnerd]

@thakis and https://github.com/llvm/llvm-project/issues/59640

clang/docs/ReleaseNotes.rst

 infrastructure are described first, followed by language-specific
 sections with improvements to Clang's support for those languages.
 
 Major New Features
 ------------------
 
 Bug Fixes
 ---------
+- ``stdatomic.h`` will use the internal declarations when targeting pre-C++-23
+  on Windows platforms as the MSVC support requires newer C++ standard.
 - Correct ``_Static_assert`` to accept the same set of extended integer
   constant expressions as is accpted in other contexts that accept them.
   This fixes `Issue 57687 <https://github.com/llvm/llvm-project/issues/57687>`_.
 - Fixes an accepts-invalid bug in C when using a ``_Noreturn`` function
   specifier on something other than a function declaration. This fixes
   `Issue 56800 <https://github.com/llvm/llvm-project/issues/56800>`_.
 - Fix `#56772 <https://github.com/llvm/llvm-project/issues/56772>`_ - invalid
   destructor names were incorrectly accepted on template classes.

clang/lib/Headers/stdatomic.h

 #ifndef __CLANG_STDATOMIC_H
 #define __CLANG_STDATOMIC_H
 
 /* If we're hosted, fall back to the system's stdatomic.h. FreeBSD, for
  * example, already has a Clang-compatible stdatomic.h header.
  *
  * Exclude the MSVC path as well as the MSVC header as of the 14.31.30818
  * explicitly disallows `stdatomic.h` in the C mode via an `#error`.  Fallback
- * to the clang resource header until that is fully supported.
+ * to the clang resource header until that is fully supported.  The
+ * `stdatomic.h` header requires C++ 23 or newer.
  */
 #if __STDC_HOSTED__ &&                                                         \
-    __has_include_next(<stdatomic.h>) && !(defined(_MSC_VER) && !defined(__cplusplus))
+    __has_include_next(<stdatomic.h>) &&                                       \
+    !(defined(_MSC_VER) && defined(__cplusplus) && __cplusplus < 202002L)
 # include_next <stdatomic.h>
 #else
 
 #include <stddef.h>
 #include <stdint.h>
 
 #ifdef __cplusplus
 extern "C" {