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3483. transform_view::iterator's difference is overconstrained

Section: 25.7.9.3 [range.transform.iterator], 25.7.23.3 [range.elements.iterator] Status: C++23 Submitter: Casey Carter Opened: 2020-09-04 Last modified: 2023-11-22

Priority: 0

View all other issues in [range.transform.iterator].

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Discussion:

The difference operation for transform_view::iterator is specified in 25.7.9.3 [range.transform.iterator] as:

friend constexpr difference_type operator-(const iterator& x, const iterator& y)
  requires random_access_range<Base>;

-22- Effects: Equivalent to: return x.current_ - y.current_;

The member current_ is an iterator of type iterator_t<Base>, where Base is V for transform_view<V, F>::iterator<false> and const V for transform_view<V, F>::iterator<true>. The difference of iterators that appears in the above Effects: element is notably well-defined if their type models sized_sentinel_for<iterator_t<Base>, iterator_t<Base>> which random_access_range<Base> refines. This overstrong requirement seems to be simply the result of an oversight; it has been present since P0789R0, without — to my recollection — ever having been discussed. We should relax this requirement to provide difference capability for transform_view's iterators whenever the underlying iterators do.

[2020-09-08; Reflector discussion]

During reflector discussions it was observed that elements_view::iterator has the same issue and the proposed wording has been extended to cover this template as well.

[2020-09-13; Reflector prioritization]

Set priority to 0 and status to Tentatively Ready after seven votes in favour during reflector discussions.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 25.7.9.3 [range.transform.iterator] as indicated:

   namespace std::ranges {
     template<input_range V, copy_constructible F>
       requires view<V> && is_object_v<F> &&   
                regular_invocable<F&, range_reference_t<V>> &&
                can-reference<invoke_result_t<F&, range_reference_t<V>>>
     template<bool Const>
     class transform_view<V, F>::iterator {
     public:
       […]
       friend constexpr iterator operator-(iterator i, difference_type n)
         requires random_access_range<Base>;
       friend constexpr difference_type operator-(const iterator& x, const iterator& y)
         requires random_access_range<Base>sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;
       […]
     };
   }
   

   […]

   friend constexpr difference_type operator-(const iterator& x, const iterator& y)
     requires random_access_range<Base>sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;
   

   -22- Effects: return x.current_ - y.current_;

2. Modify 25.7.23.3 [range.elements.iterator] as indicated:

   namespace std::ranges {
     template<input_range V, size_t N>
       requires view<V> && has-tuple-element<range_value_t<V>, N> &&
                has-tuple-element<remove_reference_t<range_reference_t<V>>, N>
     template<bool Const>
     class elements_view<V, N>::iterator {  // exposition only
       […]
       friend constexpr iterator operator-(iterator x, difference_type y)
         requires random_access_range<Base>;
       friend constexpr difference_type operator-(const iterator& x, const iterator& y)
         requires random_access_range<Base>sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;
     };
   }
   

   […]

   constexpr difference_type operator-(const iterator& x, const iterator& y)
     requires random_access_range<Base>sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;
   

   -21- Effects: return x.current_ - y.current_;