[counted.iter.cmp]

template<common_with<I> I2>
  friend constexpr bool operator=(
    const counted_iterator& x, const counted_iterator<I2>& y);

Preconditions: x and y refer to elements of the same sequence ([counted.iterator]).

Effects: Equivalent to: return x.length == y.length;

friend constexpr bool operator=(
  const counted_iterator& x, default_sentinel_t) noexcept;

Effects: Equivalent to: return x.length == 0;

template<common_with<I> I2>
  friend constexpr strong_ordering operator<=>(
    const counted_iterator& x, const counted_iterator<I2>& y);

Preconditions: x and y refer to elements of the same sequence ([counted.iterator]).

[Note 1:

The argument order in the Effects: element is reversed because length counts down, not up.

— end note]

24 Iterators library [iterators]