[clang] Allow weak/selectany external definitions in header units. (PR #162713)

[via cfe-commits]

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@@ -13822,7 +13822,10 @@ void Sema::AddInitializerToDecl(Decl *RealDecl, Expr *Init, bool DirectInit) {
       !VDecl->isInvalidDecl() && VDecl->isThisDeclarationADefinition() &&
       VDecl->getFormalLinkage() == Linkage::External && !VDecl->isInline() &&
       !VDecl->isTemplated() && !isa<VarTemplateSpecializationDecl>(VDecl) &&
-      !VDecl->getInstantiatedFromStaticDataMember()) {
+      !VDecl->getInstantiatedFromStaticDataMember() &&
+      // The intent of the above section seems to be to prevent ODR issues.
+      // Weak symbols do the same but are not part of the c++ spec.
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akrieger wrote:

https://eel.is/c++draft/module.import#6
I could import the exact phrasing `A header unit shall not contain a definition of a non-inline function or variable whose name has external linkage.`  I guess? I will iterate on it and see if I find something I like.

https://github.com/llvm/llvm-project/pull/162713