iota_view::iterator has wrong iterator category

[ericniebler]

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).

Proposed Resolution

Change the synopsis of iota_view::iterator in [range.iota.iterator] as follows:

namespace std::ranges {
  template<class W, class Bound>
  struct iota_view<W, Bound>::iterator {
  private:
    W value_ = W(); // exposition only
  public:
-   using iterator_category = see below ;
+   using iterator_concept = see below ;
+   using iterator_category = input_iterator_tag;
    using value_type = W;

Change [range.iota.iterator]/p1 as follows:

iterator::iterator_categoryconcept is defined as follows:

• If W models advanceable, then iterator_categoryconcept is random_access_iterator_tag.
• Otherwise, if W models decrementable, then iterator_categoryconcept is bidirectional_iterator_tag.
• Otherwise, if W models incrementable, then iterator_categoryconcept is forward_iterator_tag.
• Otherwise, iterator_categoryconcept is input_iterator_tag.

[morinmorin]

And transform_view has the same problem in general...?