3500. join_view::iterator::operator->() is bogus

Section: 25.7.14.3 [range.join.iterator] Status: C++23 Submitter: Michael Schellenberger Costa Opened: 2020-11-15 Last modified: 2023-11-22

Priority: 0

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View all issues with C++23 status.

Discussion:

There seems to be a copy & paste error in the join_view::iterator::operator->() specification. According to 25.7.14.3 [range.join.iterator] p8 it is specified as:

constexpr iterator_t<Base> operator->() const
  requires has-arrow<iterator_t<Base>> && copyable<iterator_t<Base>>;

-8- Effects: Equivalent to return inner_;

Now inner_ is of type iterator_t<range_reference_t<Base>>. So it is unclear how that should be converted to iterator_t<Base>, or why the constraints concern the outer iterator type iterator_t<Base>. On the other hand returning outer_ would not make any sense here.

As far as I can tell we should replace iterator_t<Base> with iterator_t<range_reference_t<Base>> so that the new specification would read

constexpr iterator_t<range_reference_t<Base>> operator->() const
 requires has-arrow<iterator_t<range_reference_t<Base>>> 
   && copyable<iterator_t<range_reference_t<Base>>>;

-8- Effects: Equivalent to return inner_;

Generally it would help readability of the specification a lot if we would introduce some exposition only aliases:

using OuterIter = iterator_t<Base>; //exposition-only
using InnerIter = iterator_t<range_reference_t<Base>> //exposition-only

and use them throughout join_view::iterator.

[2020-11-21; Reflector prioritization]

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

[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4868.

1. Modify 25.7.14.3 [range.join.iterator], class template join_view::iterator synopsis, as indicated:

   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>>)
   struct join_view<V>::iterator {
   private:
     using Parent = // exposition only
       conditional_t<Const, const join_view, join_view>;
     using Base = conditional_t<Const, const V, V>; // exposition only
     using OuterIter = iterator_t<Base>; //exposition-only
     using InnerIter = iterator_t<range_reference_t<Base>> //exposition-only
     static constexpr bool ref-is-glvalue = // exposition only
       is_reference_v<range_reference_t<Base>>;
     OuterIteriterator_t<Base> outer_ = OuterIteriterator_t<Base>(); // exposition only
     InnerIteriterator_t<range_reference_t<Base>> inner_ = // exposition only
       InnerIteriterator_t<range_reference_t<Base>>();
     Parent* parent_ = nullptr; // exposition only
     constexpr void satisfy(); // exposition only
   public:
     […]
     iterator() = default;
     constexpr iterator(Parent& parent, OuterIteriterator_t<Base> outer);
     constexpr iterator(iterator<!Const> i)
       requires Const &&
                convertible_to<iterator_t<V>, OuterIteriterator_t<Base>> &&
                convertible_to<iterator_t<InnerRng>,
                               InnerIteriterator_t<range_reference_t<Base>>>;
   
     constexpr decltype(auto) operator*() const { return *inner_; }
     
     constexpr InnerIteriterator_t<Base> operator->() const
       requires has-arrow<InnerIteriterator_t<Base>> 
         && copyable<InnerIteriterator_t<Base>>;
     
     constexpr iterator& operator++();
     […]
   

   […]

   constexpr void satisfy(); // exposition only
   

   -5- Effects: Equivalent to:

   […]
   if constexpr (ref-is-glvalue)
     inner_ = InnerIteriterator_t<range_reference_t<Base>>();
   

   constexpr iterator(Parent& parent, OuterIteriterator_t<Base> outer);
   

   […]

   constexpr iterator(iterator<!Const> i)
     requires Const &&
              convertible_to<iterator_t<V>, OuterIteriterator_t<Base>> &&
              convertible_to<iterator_t<InnerRng>,
                             InnerIteriterator_t<range_reference_t<Base>>>;
   

   […]

   constexpr InnerIteriterator_t<Base> operator->() const
     requires has-arrow<InnerIteriterator_t<Base>> 
       && copyable<InnerIteriterator_t<Base>>;
   

   -8- Effects: Equivalent to return inner_;