Add contiguous iterator utilities. How about is_contiguous_iterator and as_contiguous_range:

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  template<typename I, typename S,
      CONCEPT_REQUIRES_(RandomAccessIterator<I>() &&
                        SizedSentinel<S, I>() &&
                        is_contiguous_iterator<I>())>
  iterator_range<std::add_pointer_t<reference_t<I>>>
  as_contiguous_range(I begin, S end)
  {
      if(begin == end)
          return {nullptr, nullptr};
      else
          return {addressof(*begin), addressof(*begin) + (end - begin)};
  }

• Make inplace_merge work with forward iterators
• Make the sorting algorithms work with forward iterators
• Study the impact of allowing ForwardIterator to return proxies

• counted iterators (subtractable regardless of traversal category)
• repeat_view iterators (not subtractable but could be random access otherwise)
• infinite ranges (only countable with an infinite precision integer which we lack)