has_member type trait

#include <iostream>
#include <type_traits>

#include <cstdlib>

namespace std
{

template < typename ... > using void_t = void;

}

struct tag
{

    std::size_t id_;

};

template< typename type, typename = void >
struct is_tagged
        : std::false_type
{

};

template< typename type >
struct is_tagged< type, std::void_t< decltype(type::tag_) > >
        : std::is_same< decltype(type::tag_), tag >
{

};

struct A {};

static_assert(!is_tagged< A >{});

struct B { tag tag_; };

static_assert(is_tagged< B >{});

struct C { int tag_; };

static_assert(!is_tagged< C >{});

int
main()
{

    return EXIT_SUCCESS;
}