gistfile1.cpp

namespace detail {

template <class...>
struct void_type
{
    using type = void;
};

template <class... T>
using void_t = typename void_type<T...>::type;

template <class, class = void>
struct has_value_type : std::false_type
{
};

template <class T>
struct has_value_type <T,
    void_t<typename T::value_type>
        > : std::true_type
{
};

}