[clang][ASTImporter] Fix import of SubstTemplateTypeParmType in retur…

…n type of function. (#69724)

Import of a function with `auto` return type that is expanded to a
`SubstTemplateTypeParmType` could fail if the function itself is the
template specialization where the parameter was replaced.

clang/lib/AST/ASTImporter.cpp

@@ -3513,6 +3513,14 @@ class IsTypeDeclaredInsideVisitor
     return {};
   }
 
+  std::optional<bool>
+  VisitSubstTemplateTypeParmType(const SubstTemplateTypeParmType *T) {
+    // The "associated declaration" can be the same as ParentDC.
+    if (isAncestorDeclContextOf(ParentDC, T->getAssociatedDecl()))
+      return true;
+    return {};
+  }
+
   std::optional<bool> VisitConstantArrayType(const ConstantArrayType *T) {
     if (T->getSizeExpr() && isAncestorDeclContextOf(ParentDC, T->getSizeExpr()))
       return true;
@@ -3573,6 +3581,8 @@ class IsTypeDeclaredInsideVisitor
 };
 } // namespace
 
+/// This function checks if the function has 'auto' return type that contains
+/// a reference (in any way) to a declaration inside the same function.
 bool ASTNodeImporter::hasAutoReturnTypeDeclaredInside(FunctionDecl *D) {
   QualType FromTy = D->getType();
   const auto *FromFPT = FromTy->getAs<FunctionProtoType>();

clang/unittests/AST/ASTImporterTest.cpp

@@ -6790,10 +6790,13 @@ TEST_P(ASTImporterOptionSpecificTestBase,
 }
 
 struct ImportAutoFunctions : ASTImporterOptionSpecificTestBase {
-  void testImport(llvm::StringRef Code, clang::TestLanguage Lang = Lang_CXX14) {
+  void testImport(llvm::StringRef Code, clang::TestLanguage Lang = Lang_CXX14,
+                  bool FindLast = false) {
     Decl *FromTU = getTuDecl(Code, Lang, "input0.cc");
-    FunctionDecl *From = FirstDeclMatcher<FunctionDecl>().match(
-        FromTU, functionDecl(hasName("foo")));
+    FunctionDecl *From = FindLast ? LastDeclMatcher<FunctionDecl>().match(
+                                        FromTU, functionDecl(hasName("foo")))
+                                  : FirstDeclMatcher<FunctionDecl>().match(
+                                        FromTU, functionDecl(hasName("foo")));
 
     FunctionDecl *To = Import(From, Lang);
     EXPECT_TRUE(To);
@@ -7232,6 +7235,20 @@ TEST_P(ImportAutoFunctions, ReturnWithTypeInSwitch) {
       Lang_CXX17);
 }
 
+TEST_P(ImportAutoFunctions, ReturnWithAutoTemplateType) {
+  testImport(
+      R"(
+      template<class T>
+      struct S {};
+      template<class T>
+      auto foo() {
+        return S<T>{};
+      }
+      auto a = foo<int>();
+      )",
+      Lang_CXX14, /*FindLast=*/true);
+}
+
 struct ImportSourceLocations : ASTImporterOptionSpecificTestBase {};
 
 TEST_P(ImportSourceLocations, PreserveFileIDTreeStructure) {