[clang] [clang][modules] giving the __stddef_ headers their own modules can cause redeclaration errors with -fbuiltin-headers-in-system-modules (PR #84127)

[Volodymyr Sapsai via cfe-commits]

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@@ -7,6 +7,11 @@
  *===-----------------------------------------------------------------------===
  */
 
-#ifndef offsetof
+/*
+ * When -fbuiltin-headers-in-system-modules is set this is a non-modular header
+ * and needs to behave as if it was textual.
+ */
+#if !defined(offsetof) ||                                                      \
+    (__has_feature(modules) && !__building_module(_Builtin_stddef))
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vsapsai wrote:

I was thinking about `offsetof` being defined in some header, not in the intended system one. And then we [transitively] include "\_\_stddef_offsetof.h". Does it mean that in this case "\_\_stddef_offsetof.h" is processed only when building module "_Builtin_stddef"?

https://github.com/llvm/llvm-project/pull/84127