fix GetFunctionReturnType for templated class's method (#624)

Author: Vipul-Cariappa

lib/CppInterOp/CppInterOp.cpp

@@ -913,12 +913,22 @@ TCppType_t GetFunctionReturnType(TCppFunction_t func) {
   auto* D = (clang::Decl*)func;
   if (auto* FD = llvm::dyn_cast_or_null<clang::FunctionDecl>(D)) {
     QualType Type = FD->getReturnType();
-    if (Type->isUndeducedAutoType() && IsTemplatedFunction(FD) &&
-        !FD->isDefined()) {
+    if (Type->isUndeducedAutoType()) {
+      bool needInstantiation = false;
+      if (IsTemplatedFunction(FD) && !FD->isDefined())
+        needInstantiation = true;
+      if (auto* MD = llvm::dyn_cast<clang::CXXMethodDecl>(FD)) {
+        if (IsTemplateSpecialization(MD->getParent()))
+          needInstantiation = true;
+      }
+
+      if (needInstantiation) {
 #ifdef CPPINTEROP_USE_CLING
-      cling::Interpreter::PushTransactionRAII RAII(&getInterp());
+        cling::Interpreter::PushTransactionRAII RAII(&getInterp());
 #endif
-      getSema().InstantiateFunctionDefinition(SourceLocation(), FD, true, true);
+        getSema().InstantiateFunctionDefinition(SourceLocation(), FD, true,
+                                                true);
+      }
       Type = FD->getReturnType();
     }
     return Type.getAsOpaquePtr();

unittests/CppInterOp/FunctionReflectionTest.cpp

@@ -385,6 +385,11 @@ TEST(FunctionReflectionTest, GetFunctionReturnType) {
     template<class ...T> auto rttest_make_tlist(T ... args) {
       return RTTest_TemplatedList<T...>{};
     }
+
+    template<typename T>
+    struct Klass {
+      auto func(T t) { return t; }
+    };
     )";
 
   GetAllTopLevelDecls(code, Decls, true);
@@ -430,6 +435,11 @@ TEST(FunctionReflectionTest, GetFunctionReturnType) {
       Cpp::GetTypeAsString(Cpp::GetFunctionReturnType(
           Cpp::BestOverloadFunctionMatch(candidates, explicit_args, args))),
       "RTTest_TemplatedList<int, double>");
+
+  std::vector<Cpp::TemplateArgInfo> args2 = {C.DoubleTy.getAsOpaquePtr()};
+  EXPECT_EQ(Cpp::GetTypeAsString(Cpp::GetFunctionReturnType(Cpp::GetNamed(
+                "func", Cpp::InstantiateTemplate(Decls[15], args2.data(), 1)))),
+            "double");
 }
 
 TEST(FunctionReflectionTest, GetFunctionNumArgs) {