std::experimental::ranges::Readable - cppreference.com

template< class In >
concept bool Readable =
    requires {
        typename ranges::value_type_t<In>;
        typename ranges::reference_t<In>;
        typename ranges::rvalue_reference_t<In>;
    } &&
    CommonReference<ranges::reference_t<In>&&, ranges::value_type_t<In>&> &&
    CommonReference<ranges::reference_t<In>&&, ranges::rvalue_reference_t<In>&&> &&
    CommonReference<ranges::rvalue_reference_t<In>&&, const ranges::value_type_t<In>&>;