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3947. Unexpected constraints on adjacent_transform_view::base()

Section: 25.7.28.2 [range.adjacent.transform.view] Status: WP Submitter: Bo Persson Opened: 2023-06-17 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with WP status.

Discussion:

In section 25.7.28.2 [range.adjacent.transform.view] the class ranges::adjacent_transform_view got two new base() members from 3848(i).

The first one looks like 

constexpr V base() const & requires copy_constructible<InnerView>
{ return inner_.base(); }

Here the requirement is that InnerView is copy constructible, when it in fact returns an object of type V. That seems odd.

I would expect the constraint to instead be copy_constructible<V>.

[2023-10-27; Reflector poll]

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

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4950.

  1. Modify 25.7.28.2 [range.adjacent.transform.view], class template adjacent_transform_view synopsis, as indicated:

    namespace std::ranges {
  template<forward_range V, move_constructible F, size_t N>
    requires view<V> && (N > 0) && is_object_v<F> &&
      regular_invocable<F&, REPEAT(range_reference_t<V>, N)...> &&
      can-reference<invoke_result_t<F&, REPEAT(range_reference_t<V>, N)...>>
  class adjacent_transform_view : public view_interface<adjacent_transform_view<V, F, N>> {
    […]
    adjacent_view<V, N> inner_; // exposition only
    
    using InnerView = adjacent_view<V, N>; // exposition only
    […]
  public:
    […]
    constexpr V base() const & requires copy_constructible<VInnerView> { return inner_.base(); }
    constexpr V base() && { return std::move(inner_).base(); }
    […]
  };
}