[PATCH] D130419: Use `<stdatomic.h>` with MSVC and C++

[Igor Zhukov via Phabricator via cfe-commits]

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and use fallback only for C.

It fixes the isssue with clang-cl:

  #include <stdatomic.h>
  #include <stdbool.h>
  #ifdef __cplusplus
  #include <atomic>
  using namespace std;
  #endif
  
  int main() {
      atomic_bool b = true;
  }



  $ clang-cl /TC main.cpp
  # works

  $ clang-cl /TP /std:c++20 main.cpp
  
  stdatomic.h(70,6): error: conflicting types for 'atomic_thread_fence'
  void atomic_thread_fence(memory_order);
       ^
  atomic(166,24): note: previous definition is here
  extern "C" inline void atomic_thread_fence(const memory_order _Order) noexcept {
  
  ...
  
  fatal error: too many errors emitted, stopping now [-ferror-limit=]
  20 errors generated.

Many errors but 
`<stdatomic.h>` has many macros to built-in functions.

  #define atomic_thread_fence(order) __c11_atomic_thread_fence(order)

and MSVC `<atomic>` has real functions.
and the built-in functions are redefined.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D130419

Files:
  clang/lib/Headers/stdatomic.h


Index: clang/lib/Headers/stdatomic.h
===================================================================
--- clang/lib/Headers/stdatomic.h
+++ clang/lib/Headers/stdatomic.h
@@ -17,7 +17,8 @@
  * explicitly disallows `stdatomic.h` in the C mode via an `#error`.  Fallback
  * to the clang resource header until that is fully supported.
  */
-#if __STDC_HOSTED__ && __has_include_next(<stdatomic.h>) && !defined(_MSC_VER)
+#if __STDC_HOSTED__ &&                                                         \
+    __has_include_next(<stdatomic.h>) && !(defined(_MSC_VER) && !defined(__cplusplus))
 # include_next <stdatomic.h>
 #else
 


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