constexpr explicit sentinel(sentinel_t<V> end);
friend constexpr bool operator==(const iterator<false>& x, const sentinel& y);
friend constexpr range_difference_t<V>
operator-(const iterator<false>& x, const sentinel& y)
requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;
friend constexpr range_difference_t<V>
operator-(const sentinel& y, const iterator<false>& x)
requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;