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

[Aaron Ballman via cfe-commits]

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@@ -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();
+  }
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AaronBallman wrote:

I think http://eel.is/c++draft/dcl.stc#example-1 has a lot of good examples that are worth looking at.

We do a lot of this checking already, starting from https://github.com/llvm/llvm-project/blob/cb8d83a77c25e529f58eba17bb1ec76069a04e90/clang/lib/Sema/SemaDecl.cpp#L4643 and the issue might relate to the FIXME at https://github.com/llvm/llvm-project/blob/cb8d83a77c25e529f58eba17bb1ec76069a04e90/clang/lib/Sema/SemaDecl.cpp#L4696

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