is_belonged

#include <assert.h>
#include <type_traits>
using namespace std;

template<bool Test, typename T = void>
using enabled_if = typename enable_if<Test, T>::type;

template<typename ThisType, typename... Types>
struct is_belong;

template<typename ThisType, typename ThatType, typename... Types>
struct is_belong
    <ThisType,
     ThatType,
     Types...> : is_belong<ThisType, Types...> {};

template<typename ThisType, typename... Types>
struct is_belong
    <ThisType,
     ThisType,
     Types...> : true_type {};

template<typename ThisType>
struct is_belong<ThisType> : false_type {};

template<typename ThisType, typename... Types>  // ДОБАВЛЕНО:
constexpr bool is_belonged() {return is_belong<ThisType, Types...>::value;}

//Identity function which works only
//for `short`, `int` and `long`:
template<typename T,
     enabled_if<is_belonged<T, short, int, long>()>... Check> // ИЗМЕНЕНО
    T I(const T& val) {
        return val;
    }

int main() {

    int n = 100;
    int n1 = I(n);
    assert(n == n1);

    double d = 100;
    //Compilation error - works fine
    double d1 = I(d);
    //assert(d == d1);
}