boost::urls::grammar::RangeRule

template<class T>
concept RangeRule = requires (T r, char const*& it, char const* end)
    {
        typename T::value_type;
        { r.first(it, end) } ‐> std::same_as<system::result<typename T::value_type>>;
        { r.next(it, end) } ‐> std::same_as<system::result<typename T::value_type>>;
    };