[clang] [clang] emit an error when the same identifier appears with both internal and external linkage in a translation unit (PR #78064)

[via cfe-commits]

================
@@ -4754,6 +4754,11 @@ void Sema::MergeVarDecl(VarDecl *New, LookupResult 
&Previous) {
     return New->setInvalidDecl();
   }
 
+  if (Old->getFormalLinkage() != New->getFormalLinkage()) {
+    Diag(New->getLocation(), diag::err_multiple_linkage) << New->getDeclName();
+    return New->setInvalidDecl();
+  }
----------------
elhewaty wrote:

@AaronBallman
> If the prior declaration specifies internal or external linkage, the linkage 
> of the identifier at the later declaration is the same as the linkage 
> specified at the prior declaration.

Then when should the declaration be an issue (https://godbolt.org/z/roE9xh76d)?
[Note: the diagnostic of clang and GCC is different.]
If the external linkage follows the previous linkage, when could the error 
happen?




https://github.com/llvm/llvm-project/pull/78064
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