template<class S, class I>
concept sentinel_for =
semiregular<S> &&
input_or_output_iterator<I> &&
weakly-equality-comparable-with<S, I>; // see [concept.equalitycomparable]
template<class S, class I>
concept sized_sentinel_for =
sentinel_for<S, I> &&
!disable_sized_sentinel_for<remove_cv_t<S>, remove_cv_t<I>> &&
requires(const I& i, const S& s) {
{ s - i } -> same_as<iter_difference_t<I>>;
{ i - s } -> same_as<iter_difference_t<I>>;
};
template<class S, class I>
inline constexpr bool disable_sized_sentinel_for = false;