[clang] [Clang][P1061] Fix template arguments in local classes (PR #121225)

[Matheus Izvekov via cfe-commits]

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@@ -4433,8 +4433,12 @@ Sema::InstantiateClassMembers(SourceLocation PointOfInstantiation,
       // No need to instantiate in-class initializers during explicit
       // instantiation.
       if (Field->hasInClassInitializer() && TSK == TSK_ImplicitInstantiation) {
+        // Handle local classes which could have substituted template params.
         CXXRecordDecl *ClassPattern =
-            Instantiation->getTemplateInstantiationPattern();
+            Instantiation->isLocalClass()
+                ? Instantiation->getInstantiatedFromMemberClass()
+                : Instantiation->getTemplateInstantiationPattern();
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mizvekov wrote:

The main point of `getTemplateInstantiationPattern` is to figure out the instantiation pattern as the name suggests, it would be better to fix the problem within it.

https://github.com/llvm/llvm-project/pull/121225