[clang] [Clang] Apply exclude_from_explicit_instantiation to dllimport/dllexport (PR #168171)

[Tomohiro Kashiwada via cfe-commits]

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@@ -19108,8 +19118,23 @@ bool Sema::DefineUsedVTables() {
         }
       }
 
-      if (IsExplicitInstantiationDeclaration)
+      if (IsExplicitInstantiationDeclaration) {
         DefineVTable = false;
+
+        // Ensure the instance of a virtual member function which is declared
+        // with `__attribute__((exclude_from_explicit_instantiation))` is
+        // accessible from the VTable.
+        for (Decl *decl : Class->decls()) {
+          auto *Method = dyn_cast<CXXMethodDecl>(decl);
+          if (!Method || !Method->isVirtual())
+            continue;
+
+          if (Method->hasAttr<ExcludeFromExplicitInstantiationAttr>()) {
+            MarkFunctionReferenced(Loc, Method);
+            DefinedAnything = true;
+          }
+        }
----------------
kikairoya wrote:

I wasn't sure if `Consumer.HandleVTable(Class);` below should be called in this case; that was not called previously. However, since the VTable is actually emitted, it might need to be called.

Surprisingly, setting `DefineVTable = true` regardless `IsExplicitInstantiationDeclaration` doesn't break any tests. 
The current construction was introduced in 88d292ccb86c108, but the commit doesn't mention MSVC ABI.
How do I check "correct" behavior?

https://github.com/llvm/llvm-project/pull/168171