[clang] [Clang] Avoid assertion failure for initialized extern aliases (PR #223124)

[Aaron Ballman via cfe-commits]

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@@ -14127,6 +14093,15 @@ void Sema::AddInitializerToDecl(Decl *RealDecl, Expr *Init, bool DirectInit) {
     return;
   }
 
+  // handleAliasAttr() runs before the initializer is attached to the VarDecl,
+  // so it cannot determine that an extern declaration with an initializer is a
+  // definition. Diagnose the conflict now that an initializer is present and
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AaronBallman wrote:

> So I'm a little unsure how this declaration should be classified in this case.

You're in good company, I was struggling with that yesterday too. :-D Because the purpose to alias is to say "this declaration is just another name for that object definition over there", I think putting the alias attribute on the static data member makes sense because that's just a declaration. But that means an out-of-line definition for it would be invalid because it stops being a declaration and starts being a definition at that point. For example, this shows that we correctly treat it as a declaration without the out-of-line definition and has the expected semantics: https://godbolt.org/z/cq9hTxdKE

CC @efriedma-quic for a second opinion on this logic though

https://github.com/llvm/llvm-project/pull/223124