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### 3291. `iota_view::iterator` has the wrong `iterator_category`

Section: 24.6.4.3 [range.iota.iterator] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-13 Last modified: 2021-02-25

Priority: 0

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Discussion:

In the old way of looking at the world, forward iterators need to return real references. Since dereferencing `iota_view`'s iterators returns by value, it cannot be a C++17 forward iterator. (It can, however, be a C++20 `forward_iterator`.) However, `iota_view`'s iterator has an `iterator_category` that (sometimes) falsely claims that it is forward or better (depending on the properties of the `weakly_incrementable` type it wraps).

[2019-10-19 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after eight positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify 24.6.4.3 [range.iota.iterator] as indicated:

   > >     namespace std::ranges {
   > >       template<class W, class Bound>
   > >       struct iota_view<W, Bound>::iterator {
   > >       private:
   > >         […]
   > >       public:
   > >         using iterator_conceptategory = see below;
   > >         using iterator_category = input_iterator_tag;
   > >         using value_type = W;
   > >         using difference_type = IOTA-DIFF-T(W);
   > >         […]
   > >       };
   > >       […]
   > >     }

   > -1- `iterator::iterator_conceptategory` is defined as follows:

   > 1. (1.1) — If `W` models `advanceable`, then `iterator_conceptategory` is `random_access_iterator_tag`.

   > 2. (1.2) — Otherwise, if `W` models `decrementable`, then `iterator_conceptategory` is `bidirectional_iterator_tag`.

   > 3. (1.3) — Otherwise, if `W` models `incrementable`, then `iterator_conceptategory` is `forward_iterator_tag`.

   > 4. (1.4) — Otherwise, `iterator_conceptategory` is `input_iterator_tag`.