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3517. join_view::iterator's iter_swap is underconstrained

Section: 26.7.14.3 [range.join.iterator] Status: WP Submitter: Casey Carter Opened: 2021-01-28 Last modified: 2021-06-07

Priority: Not Prioritized

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Discussion:

std::ranges::join_view::iterator's hidden friend iter_swap is specified in 26.7.14.3 [range.join.iterator]/16 as:

friend constexpr void iter_swap(const iterator& x, const iterator& y)
  noexcept(noexcept(ranges::iter_swap(x.inner_, y.inner_)));

-16- Effects: Equivalent to: return ranges::iter_swap(x.inner_, y.inner_);

Notably, the expression ranges::iter_swap(meow, woof) is not valid for all iterators meow and woof, or even all input iterators of the same type as is the case here. This iter_swap overload should be constrained to require the type of iterator::inner_ (iterator_t<range_reference_t<maybe-const<Const, V>>>) to satisfy indirectly_swappable. Notably this is already the case for iter_swap friends of every other iterator adaptor in the Standard Library (reverse_iterator, move_iterator, common_iterator, counted_iterator, filter_view::iterator, transform_view::iterator, and split_view::inner-iterator). The omission for join_view::iterator seems to have simply been an oversight.

[2021-03-12; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4878.

1. Modify 26.7.14.3 [range.join.iterator] as indicated:

   [Drafting note: If 3500 is accepted before this issue, it is kindly suggested to the Project Editor to apply the equivalent replacement of "iterator_t<range_reference_t<Base>>" by "InnerIter" to the newly inserted requires.

   namespace std::ranges {
     template<input_range V>
       requires view<V> && input_range<range_reference_t<V>> &&
                (is_reference_v<range_reference_t<V>> ||
                view<range_value_t<V>>)
     template<bool Const>
     struct join_view<V>::iterator {
       […]
       
       friend constexpr void iter_swap(const iterator& x, const iterator& y)
         noexcept(noexcept(ranges::iter_swap(x.inner_, y.inner_)))
         requires indirectly_swappable<iterator_t<range_reference_t<Base>>>;
     };
   }
   

   […]

   friend constexpr void iter_swap(const iterator& x, const iterator& y)
     noexcept(noexcept(ranges::iter_swap(x.inner_, y.inner_)))
     requires indirectly_swappable<iterator_t<range_reference_t<Base>>>;
   

   -16- Effects: Equivalent to: return ranges::iter_swap(x.inner_, y.inner_);