[clang] [Clang] Diagnose UB and emit error when identifier has both internal and external linkage (PR #192116)

[Aaron Ballman via cfe-commits]

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@@ -4800,6 +4800,23 @@ void Sema::MergeVarDecl(VarDecl *New, LookupResult &Previous) {
       return New->setInvalidDecl();
     }
   }
+
+  // C11 6.2.2p7:
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AaronBallman wrote:

You should cite C2y (from the latest working draft: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3854.pdf) and then mention this was UB in earlier modes. 

https://github.com/llvm/llvm-project/pull/192116