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

[Jason Rice 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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ricejasonf wrote:

The comments for that states 
```cpp
  /// Retrieve the record declaration from which this record could be
  /// instantiated. Returns null if this class is not a template instantiation.
  const CXXRecordDecl *getTemplateInstantiationPattern() const;
```

So the "pattern" as I read it, is not an instantiation itself but the template (or partial specialization). In the case of the field in  `struct L` we need the intermediate instantiated struct to get the instantiated field.

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