[clang] [clang][ASTImporter] import InstantiatedFromMember of ClassTemplateSpecializationDecl (PR #76493)

[Balázs Kéri via cfe-commits]

================
@@ -9342,6 +9342,38 @@ TEST_P(ASTImporterOptionSpecificTestBase, ImportConflictTypeAliasTemplate) {
   EXPECT_FALSE(ImportedCallable);
 }
 
+AST_MATCHER(ClassTemplateSpecializationDecl, hasInstantiatedFromMember) {
+  if (auto Instantiate = Node.getInstantiatedFrom()) {
+    if (auto *FromPartialSpecialization =
+            Instantiate.get<ClassTemplatePartialSpecializationDecl *>()) {
+      return nullptr != FromPartialSpecialization->getInstantiatedFromMember();
+    }
+  }
+  return false;
+}
+
+TEST_P(ASTImporterOptionSpecificTestBase, ImportInstantiatedFromMember) {
+  const char *Code =
+      R"(
+      template <typename> struct B {
+        template <typename, bool = false> union D;
+        template <typename T> union D<T> {};
+        D<int> d;
+      };
+      B<int> b;
+      )";
+  Decl *FromTU = getTuDecl(Code, Lang_CXX11);
+  auto *FromA = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      FromTU, classTemplateSpecializationDecl(hasName("D"),
+                                              hasInstantiatedFromMember()));
+  auto *FromPartialSpecialization =
+      cast<ClassTemplatePartialSpecializationDecl *>(
+          FromA->getInstantiatedFrom());
----------------
balazske wrote:

```suggestion
          FromD->getInstantiatedFrom());
```

https://github.com/llvm/llvm-project/pull/76493