3846. iota_view::iterator::operator- is overconstrained

Section: 25.6.4.3 [range.iota.iterator] Status: New Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2023-02-01

Priority: 3

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Discussion:

Currently, two iota_view::iterators can be subtracted only when the underlying W type models advanceable, where advanceable consists of a series of syntactic and semantic requirements similar to the random_access_iterator concept.

However, when W is an C++20 iterator type, whether it provides subtraction is irrelevant to its iterator category. In such cases, still requiring W to support a series of random access iterator-like operations seems too restrictive. Consider:

    #include <list>
    #include <ranges>

    int main() {
      std::list l{1, 2, 3, 4, 5};
      auto it = std::counted_iterator(l.begin(), l.size());
      auto r = std::views::iota(it, std::next(it, 3));
      auto sz = r.size();           // 3 as expected
      auto d = r.end() - r.begin(); // error: no match for 'operator-'
    }

We can get the correct size of iota_view by subtracting two counted_iterators, but we cannot subtract two iota_view::iterators to get their difference, even though the underlying counted_iterator already models sized_sentinel_for for itself, which is not satisfactory.

I think we should relax the constraints of iota_view::iterator::operator- to allow the above case, which also makes it compatible with iota_view::sentinel::operator-.

[2023-02-01; Reflector poll]

Set priority to 3 after reflector poll. Several P0 votes, but an objection to P0 on the basis that we don't define what it means to use sized_sentinel_for on non-iterators. Others responded that we don't need to, as we only use it with iterators, and do not intend it to be usable with anything else.

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.6.4.3 [range.iota.iterator] as indicated:

   namespace std::ranges {
     template<weakly_incrementable W, semiregular Bound>
       requires weakly-equality-comparable-with<W, Bound> && copyable<W>
     struct iota_view<W, Bound>::iterator {
     private:
       W value_ = W();             // exposition only
     public:
       […]
       friend constexpr iterator operator-(iterator i, difference_type n)
         requires advanceable<W>;
       friend constexpr difference_type operator-(const iterator& x, const iterator& y)
         requires advanceable<W> || sized_sentinel_for<W, W>;
     };
   }
   

   […]

   friend constexpr difference_type operator-(const iterator& x, const iterator& y)
     requires advanceable<W> || sized_sentinel_for<W, W>;
   

   -23- Effects: Equivalent to:

     using D = difference_type;
     if constexpr (is-integer-like<W>) {
       if constexpr (is-signed-integer-like<W>)
         return D(D(x.value_) - D(y.value_));
       else
         return (y.value_ > x.value_)
           ? D(-D(y.value_ - x.value_))
           : D(x.value_ - y.value_);
     } else {
       return x.value_ - y.value_;
     }