constexpr counted_iterator(I i, iter_difference_t<I> n);
template<class I2>
requires convertible_to<const I2&, I>
constexpr counted_iterator(const counted_iterator<I2>& x);
template<class I2>
requires assignable_from<I&, const I2&>
constexpr counted_iterator& operator=(const counted_iterator<I2>& x);