[clang] [Headers] Don't declare unreachable() from stddef.h in C++ (PR #86748)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
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@llvm/pr-subscribers-clang

Author: Ian Anderson (ian-twilightcoder)

<details>
<summary>Changes</summary>

Even if __need_unreachable is set, stddef.h should not declare unreachable() in C++ because it conflicts with the declaration in <utility>.

---
Full diff: https://github.com/llvm/llvm-project/pull/86748.diff


1 Files Affected:

- (modified) clang/lib/Headers/__stddef_unreachable.h (+4) 


``````````diff
diff --git a/clang/lib/Headers/__stddef_unreachable.h b/clang/lib/Headers/__stddef_unreachable.h
index 518580c92d3f5d..61df43e9732f8a 100644
--- a/clang/lib/Headers/__stddef_unreachable.h
+++ b/clang/lib/Headers/__stddef_unreachable.h
@@ -7,6 +7,8 @@
  *===-----------------------------------------------------------------------===
  */
 
+#ifndef __cplusplus
+
 /*
  * When -fbuiltin-headers-in-system-modules is set this is a non-modular header
  * and needs to behave as if it was textual.
@@ -15,3 +17,5 @@
     (__has_feature(modules) && !__building_module(_Builtin_stddef))
 #define unreachable() __builtin_unreachable()
 #endif
+
+#endif

``````````

</details>


https://github.com/llvm/llvm-project/pull/86748