constexpr explicit sentinel(Bound bound);
friend constexpr bool operator==(const iterator& x, const sentinel& y);
friend constexpr iter_difference_t<W> operator-(const iterator& x, const sentinel& y)
requires sized_sentinel_for<Bound, W>;
friend constexpr iter_difference_t<W> operator-(const sentinel& x, const iterator& y)
requires sized_sentinel_for<Bound, W>;