constexpr explicit sentinel(tuple<sentinel_t<maybe-const<Const, Views>>...> end);
constexpr sentinel(sentinel<!Const> i)
requires Const && (convertible_to<sentinel_t<Views>, sentinel_t<const Views>> && ...);
template<bool OtherConst>
requires (sentinel_for<sentinel_t<maybe-const<Const, Views>>,
iterator_t<maybe-const<OtherConst, Views>>> && ...)
friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);
template<bool OtherConst>
requires (sized_sentinel_for<sentinel_t<maybe-const<Const, Views>>,
iterator_t<maybe-const<OtherConst, Views>>> && ...)
friend constexpr common_type_t<range_difference_t<maybe-const<OtherConst, Views>>...>
operator-(const iterator<OtherConst>& x, const sentinel& y);
template<bool OtherConst>
requires (sized_sentinel_for<sentinel_t<maybe-const<Const, Views>>,
iterator_t<maybe-const<OtherConst, Views>>> && ...)
friend constexpr common_type_t<range_difference_t<maybe-const<OtherConst, Views>>...>
operator-(const sentinel& y, const iterator<OtherConst>& x);