C++17 + Concepts: Palindrome check for Forward Iterators

palindrome.cpp

//Complexity:
//  Time:   floor(n / 2) equality comparisons
//  Space:  floor(n / 2) recursive calls

template <ForwardIterator I>
	requires(Readable<I>)
std::pair<bool, I> palindrome_recursive(I f, DistanceType<I> n) {
    //precondition: readable_weak_range(f, n)

    if (zero(n)) return {true, f};
    if (one(n))  return {true, ++f};

    auto r = palindrome_recursive(std::next(f), n - 2);

    if ( ! r.first) return r;
    if (*f != *r.second) return {false, r.second};

    return {true, ++r.second};
}