a counted range, and slide working with counted and non-counted iterators

slide.cpp

#include <vector>
#include <utility>
#include <iostream>
#include <algorithm>
#include <range/v3/range.hpp>
using namespace ranges;

template<typename It>
class counted_range : public range_facade<counted_range<It>>
{
    friend range_core_access;
    It it_;
    iterator_difference_t<It> n_;
    auto current() const -> decltype(*it_)
        { return *it_; }
    bool done() const
        { return 0 == n_; }
    bool equal(counted_range<It> const &that) const
        { return n_ == that.n_; }
    void next() { ++it_, --n_; }
    CONCEPT_REQUIRES(BidirectionalIterator<It>())
    void prev()
        { --it_, ++n_; }
    CONCEPT_REQUIRES(RandomAccessIterator<It>())
    void advance(iterator_difference_t<It> n)
        { it_ += n, n_ -= n; }
    CONCEPT_REQUIRES(RandomAccessIterator<It>())
    iterator_difference_t<It>
    distance_to(counted_range<It> const &that) const
        { return n_ - that.n_; }
public:
    counted_range(It it, iterator_difference_t<It> n)
      : it_(it), n_(n)
    {}
};

template<typename It>
counted_range<It> counted(It it, iterator_difference_t<It> n)
{
    return {it, n};
}

struct my_vector
  : range_adaptor<my_vector, std::vector<int>>
{
    my_vector() = default;
    my_vector(std::initializer_list<int> i)
      : range_adaptor_t<my_vector>{i}
    {}
};

template<typename I,
    CONCEPT_REQUIRES_(RandomAccessIterator<I>())>
iterator_range<I> slide(I f, I l, I p)
{
    if(p < f) return {p, std::rotate(p, f, l) };
    if(l < p) return {std::rotate(f, l, p), p };
    return { f, l };
}

int main()
{
    // Make a vector
    my_vector v {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19};

    // This works:
    slide(next(v.begin(), 5), next(v.begin(), 7), next(v.begin(), 17));
    for_each(v, [](int i){std::cout << i << ' ';});
    std::cout << std::endl;

    // Make a counted range of the vector:
    auto r = counted(begin(v), distance(v));

    // This works, too
    slide(next(r.begin(), 15), next(r.begin(), 17), next(r.begin(), 5));
    for_each(r, [](int i){std::cout << i << ' ';});
    std::cout << std::endl;
}