How to check template class method return type?

I try to check in Class2 return return value type of method getGG() of class given as template parameter but my code doesn't compile. How to do it properly?

template <class T, class U>
struct hasProperMethodReturnValueType {
    static constexpr bool value = std::is_same<T, std::decltype(U.getGG())>;
};

template<class P> class Class1 {
private:
    P gg;
public: 
    Class1(P a) : gg(a) {} 
    P getGG() {
        return gg;
    }   
};

template<class A, class P> class Class3 {
private:
    P gg;
    A dd;
public: 
    Class3(P a, A r) : gg(a), dd(r) {} 
    P getGG() {
        return gg;
    }   
};

template<class G, class R> class Class2 {
    static_assert(hasProperMethodReturnValueType<G, R>::value, "Not same type");
private:
    R cc;
public:
    Class2(R r) : cc(r) {};
};

int main() {
    auto obj  = Class2<int, Class1<int> >(Class1<int>(3));
    auto obj2  = Class2<int, Class3<float, int> >(Class3<float, int>(0, 1.1));
    return 0;
}

Compilation error:

error: template argument 2 is invalid
  static constexpr bool value = std::is_same<T, std::decltype(U.getGG())>;