[clang] [C++20] [Modules] merge the type for anony enum from import and #include (PR #214121)

[Chuanqi Xu via cfe-commits]

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@@ -2667,6 +2667,22 @@ void Sema::MergeTypedefNameDecl(Scope *S, TypedefNameDecl *New,
       else
         New->setTypeSourceInfo(OldTD->getTypeSourceInfo());
 
+      // An anonymous enum is recognized as a redeclaration only when its typedef
+      // name gets merged, at which point two distinct enum types already exist.
+      // Retype the new enumerators to the old enum type, matching the typedef
+      // merge above; otherwise the merged typedef and its enumerators disagree
+      // on the type (GH213299).
+      //
+      // FIXME: The global module restriction only limits the impact of this
+      // change; relax it if the issue shows up in other contexts.
+      if (Module *M = OldTag->getOwningModule(); M && M->isGlobalModule())
+        if (auto *NewEnum = dyn_cast<EnumDecl>(NewTag))
+          if (auto *OldEnum = dyn_cast<EnumDecl>(OldTag)) {
+            QualType OldEnumType = Context.getCanonicalTagType(OldEnum);
+            for (auto *ECD : NewEnum->enumerators())
+              ECD->setType(OldEnumType);
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ChuanqiXu9 wrote:

Yeah, this is the test I am looking for. I've added this to clang/unittests/Sema/SemaModuleTest.cpp. And this is what `NewEnum->setPreviousDecl(OldEnum);` are doing here.

https://github.com/llvm/llvm-project/pull/214121