template<typename T, typename = void> // sth like T=>int or T=>double not valid
struct test : std::false_type{};

template<typename T, ???> // what should i do?
struct test<T,???> : std::true_type{};

std::cout << test<int>::value << std::endl;

class a{ int a;double b};
std::cout << test<a>::value << std::endl;