[ASTImporter] Corrected lookup at import of templated record decl

[balazske]

Summary

When a CXXRecordDecl under ClassTemplateDecl is imported, check
the templated record decl for similarity instead of the template.

[martong]

Looks good to me, but let's wait for Aleksei's approval and comments.

[a.sidorin]

LGTM too, thank you!
Do you need someone to commit this for you?

[balazske]

Yes, I have no commit permissions.

[martong]

Balazs, I'll commit it for you in an hour.

lib/AST/ASTImporter.cpp

for (auto *FoundDecl : FoundDecls) {
       if (!FoundDecl->isInIdentifierNamespace(IDNS))
         continue;
       
       Decl *Found = FoundDecl;
       if (auto *Typedef = dyn_cast<TypedefNameDecl>(Found)) {
         if (const auto *Tag = Typedef->getUnderlyingType()->getAs<TagType>())
           Found = Tag->getDecl();
       }
-      
+      if (D->getDescribedTemplate()) {
+        if (auto *Template = dyn_cast<ClassTemplateDecl>(Found))
+          Found = Template->getTemplatedDecl();
+        else
+          continue;
+      }
+
       if (auto *FoundRecord = dyn_cast<RecordDecl>(Found)) {
         if (!SearchName) {
           // If both unnamed structs/unions are in a record context, make sure
           // they occur in the same location in the context records.
           if (Optional<unsigned> Index1 =
                   StructuralEquivalenceContext::findUntaggedStructOrUnionIndex(
                       D)) {
             if (Optional<unsigned> Index2 = StructuralEquivalenceContext::

unittests/AST/ASTImporterTest.cpp

TEST(ImportExpr, DependentSizedArrayType) {
   testImport("template<typename T, int Size> class declToImport {"
              "  T data[Size];"
              "};",
              Lang_CXX, "", Lang_CXX, Verifier,
              classTemplateDecl(has(cxxRecordDecl(
                  has(fieldDecl(hasType(dependentSizedArrayType())))))));
 }
 
+TEST_P(ASTImporterTestBase, ImportOfTemplatedDeclOfClassTemplateDecl) {
+  Decl *FromTU = getTuDecl("template<class X> struct S{};", Lang_CXX);
+  auto From =
+      FirstDeclMatcher<ClassTemplateDecl>().match(FromTU, classTemplateDecl());
+  ASSERT_TRUE(From);
+  auto To = cast<ClassTemplateDecl>(Import(From, Lang_CXX));
+  ASSERT_TRUE(To);
+  Decl *ToTemplated = To->getTemplatedDecl();
+  Decl *ToTemplated1 = Import(From->getTemplatedDecl(), Lang_CXX);
+  EXPECT_TRUE(ToTemplated1);
+  EXPECT_EQ(ToTemplated1, ToTemplated);
+}
+
+TEST_P(ASTImporterTestBase, ImportCorrectTemplatedDecl) {
+  auto Code =
+        R"(
+        namespace x {
+          template<class X> struct S1{};
+          template<class X> struct S2{};
+          template<class X> struct S3{};
+        }
+        )";
+  Decl *FromTU = getTuDecl(Code, Lang_CXX);
+  auto FromNs =
+      FirstDeclMatcher<NamespaceDecl>().match(FromTU, namespaceDecl());
+  auto ToNs = cast<NamespaceDecl>(Import(FromNs, Lang_CXX));
+  ASSERT_TRUE(ToNs);
+  auto From =
+      FirstDeclMatcher<ClassTemplateDecl>().match(FromTU,
+                                                  classTemplateDecl(
+                                                      hasName("S2")));
+  auto To =
+      FirstDeclMatcher<ClassTemplateDecl>().match(ToNs,
+                                                  classTemplateDecl(
+                                                      hasName("S2")));
+  ASSERT_TRUE(From);
+  ASSERT_TRUE(To);
+  auto ToTemplated = To->getTemplatedDecl();
+  auto ToTemplated1 =
+      cast<CXXRecordDecl>(Import(From->getTemplatedDecl(), Lang_CXX));
+  EXPECT_TRUE(ToTemplated1);
+  ASSERT_EQ(ToTemplated1, ToTemplated);
+}
+
 TEST_P(ASTImporterTestBase, DISABLED_ImportFunctionWithBackReferringParameter) {
   Decl *From, *To;
   std::tie(From, To) = getImportedDecl(
       R"(
       template <typename T> struct X {};
 
       void declToImport(int y, X<int> &x) {}