[libcxx-commits] [libcxx] [libc++] Fix diagnostic for <stdatomic.h> before C++23 (PR #83351)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libcxx

Author: Louis Dionne (ldionne)

<details>
<summary>Changes</summary>

We normally try to issue a reasonable diagnostic when mixing <stdatomic.h> and <atomic> before C++23. However, after granularizing the <atomic> header, the check and the #error message was moved to *after* the point where mixing both causes problems. When mixing both headers, we would hence get the diagnostic burried under a pile of previous diagnostics in e.g. __atomic/kill_dependency.h.

This patch moves the check earlier to restore the intended behavior. It also switches from `#ifdef kill_dependency` to an explicit check of the inclusion of the header and the Standard version, which seems to be more reliable than checking whether a macro is defined.

---
Full diff: https://github.com/llvm/llvm-project/pull/83351.diff


1 Files Affected:

- (modified) libcxx/include/atomic (+4-4) 


``````````diff
diff --git a/libcxx/include/atomic b/libcxx/include/atomic
index 2e8f5b521a55eb..ab6fc566aabc02 100644
--- a/libcxx/include/atomic
+++ b/libcxx/include/atomic
@@ -587,6 +587,10 @@ template <class T>
 
 */
 
+#if _LIBCPP_STD_VER < 23 && defined(_LIBCPP_STDATOMIC_H)
+#  error <atomic> is incompatible with <stdatomic.h> before C++23. Please compile with -std=c++23.
+#endif
+
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__atomic/aliases.h>
 #include <__atomic/atomic.h>
@@ -613,10 +617,6 @@ template <class T>
 #  error <atomic> is not implemented
 #endif
 
-#ifdef kill_dependency
-#  error <atomic> is incompatible with <stdatomic.h> before C++23. Please compile with -std=c++23.
-#endif
-
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
 #  include <cmath>
 #  include <compare>

``````````

</details>


https://github.com/llvm/llvm-project/pull/83351