pair_copy.h

struct __nat {};

pair& operator=(typename conditional<
                        is_copy_assignable<first_type>::value &&
                        is_copy_assignable<second_type>::value,
                    pair, __nat>::type const& __p)
    noexcept(is_nothrow_copy_assignable<first_type>::value &&
             s_nothrow_copy_assignable<second_type>::value)
{
    first = __p.first;
    second = __p.second;
    return *this;
}