type_traits

template<class T, T v>
struct integral_constant {
    static constexpr T value = v;
    using value_type = T;
    using type = integral_constant; // using injected-class-name
    constexpr operator value_type() const noexcept { return value; }
    constexpr value_type operator()() const noexcept { return value; } //since c++14
};
using false_type = integral_constant<bool, false>;
using true_type = integral_constant<bool, true>;

template<class T, class U>
struct is_same : false_type {};
template<class T>
struct is_same<T, T> : true_type {};