boost::urls::grammar::see_below::is_charset<T, void_t<decltype(std::declval<bool&>() =

template<class T>
struct is_charset<T, void_t<decltype(std::declval<bool&>() =
        std::declval<T const&>().operator()(
            std::declval<char>()))>>
    : std::true_type;