[clang] [C++20] [Modules] [Itanium ABI] Generate the vtable in the module unit of dynamic classes (PR #75912)

[Eli Friedman via cfe-commits]

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@@ -6853,6 +6853,13 @@ void CodeGenModule::EmitTopLevelDecl(Decl *D) {
         if (ES->hasExternalDefinitions(D) == ExternalASTSource::EK_Never)
           DI->completeUnusedClass(*CRD);
     }
+    // If we're emitting a dynamic class from the importable module we're
+    // emitting, we always need to emit the virtual table according to the ABI
+    // requirement.
+    if (CRD->getDefinition() && CRD->isDynamicClass() &&
+        CRD->isInCurrentModuleUnit())
+      EmitVTable(CRD);
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efriedma-quic wrote:

You mean Sema::DefineUsedVTables doesn't run at all in such cases?

My primary concern here is that having multiple overlapping codepaths is hard to maintain.  If Sema::DefineUsedVTables can't actually force the definition in all the relevant cases, we should just eliminate HandleVTable.

https://github.com/llvm/llvm-project/pull/75912