constexpr explicit sentinel(sentinel_t<Base> end, const Pred* pred);
constexpr sentinel(sentinel<!Const> s)
requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;
friend constexpr bool operator==(const iterator_t<Base>& x, const sentinel& y);
template<bool OtherConst = !Const>
requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
friend constexpr bool operator==(const iterator_t<maybe-const<OtherConst, V>>& x,
const sentinel& y);