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