is_element_of_sequence

#include <utility>

namespace stdext
{

namespace details
{

template<typename T, T... elems>
struct is_element_of_sequence_impl
{
    static inline bool _do(const T& value, std::integer_sequence<T, elems...>) noexcept;
};

template<typename T, T elem, T... elems>
struct is_element_of_sequence_impl<T, elem, elems...>
{
    static inline bool _do(const T& value, std::integer_sequence<T, elem, elems...>) noexcept
    {
        if (elem != value)
        {
             using seq = std::integer_sequence<T, elems...>;
             return is_elemenet_of_sequence_impl<T, elems...>::_do(value, seq{});
        }
        return true;
    }
};

template<typename T>
struct is_element_of_sequence_impl<T>
{
    static inline bool _do(const T& value, std::integer_sequence<T>)
    {
        return false;
    }
};

} // namespace details

template<typename T, T... elems>
bool is_element_of_sequence(const T& t, std::integer_sequence<T, elems...>)
{
    using seq = std::integer_sequence<T, elems...>;
    return details::is_element_of_sequence_impl<T, elems...>::_do(t, seq{});
}

} // namespace stdext

// Example

std::integer_sequence<int, 1, 2, 3, 4, 5> seq;
int i1 = 3;
int i2 = 10;

stdext::is_element_of_sequence(i1, seq); // return true
stdext::is_element_of_sequence(i2, seq); // return false