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### 3492. Minimal improvements to `elements_view::iterator`

Section: 26.7.22.3 [range.elements.iterator] Status: WP Submitter: Michael Schellenberger Costa Opened: 2020-10-28 Last modified: 2021-02-26

Priority: 0

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Discussion:

During code review of `elements_view` for MSVC-STL we found two issues that should be easily addressed:

1. `elements_view::iterator` constraints both `operator++(int)` member functions

   >     constexpr void operator++(int) requires (!forward_range<Base>);
   >     constexpr iterator operator++(int) requires forward_range<Base>;

   However, given that a constrained method would be preferred we only need to constrain one of those. The proposal would be to remove the constraint from the `void` returning overload and change the declaration to

   >     constexpr void operator++(int);
   >     constexpr iterator operator++(int) requires forward_range<Base>;

2. `elements_view::iterator operator-` is constrained as follows:

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

   However, that requires its base to have `operator-` defined. We should change the constraint to `sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>`:

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

[2020-11-01; Daniel comments]

Bullet (2) of the discussion has already been resolved by LWG 3483, it has therefore been omitted from the proposed wording below.

[2020-11-15; Reflector prioritization]

Set priority to 0 and status to Tentatively Ready after five 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.

> This wording intentionally only touches `operator++(int)` and not `operator-`, see the 2020-11-01 comment for the reason why.

1. Modify 26.7.22.3 [range.elements.iterator], class template `elements_view::iterator` synopsis, as indicated:

   > >     […]
   > >     constexpr iterator& operator++();
   > >     constexpr void operator++(int) requires (!forward_range<Base>);
   > >     constexpr iterator operator++(int) requires forward_range<Base>;
   > >     […]

   > […]

   >     constexpr void operator++(int) requires (!forward_range<Base>);

   > > -6- Effects: Equivalent to: `++current_`.

   >     constexpr iterator operator++(int) requires forward_range<Base>;

   > […]