constexpr explicit sentinel(Parent& parent);
constexpr sentinel(sentinel<!Const> s)
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);