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