Fix transform_view::iterator::iterator_category when iter_reference_t is not an lvalue ref

[morinmorin]

Since iter_reference_t<transform_view::iterator> can be a type that is not an lvalue-reference, the current wording on its iterator_category needs to be fixed.

Proposed Resolution

Change [range.transform.iterator] p2 as follows:

+ \tcode{iterator::iterator_category} is defined as follows:
  — Let `C` denote the type `iterator_traits<iterator_t<Base>>::iterator_category`.
+ — If `is_lvalue_reference_v<iter_reference_t<iterator_t<Base>>>` is `true`,
    — If `C` models `derived_from<contiguous_iterator_tag>`, 
      then `iterator_category` denotes `random_access_iterator_tag`. 
    — Otherwise, `iterator_category` denotes `C`.
+ — Otherwise, `iterator_category` denotes `input_iterator_tag`.

Making iterator_category unconditionally input_iterator_tag would be the safest fix, but I think the above fix works fine.

(I'm planning to submit this to LWG in a few days.)

[CaseyCarter]

A model of input_iterator may fail to meet the Cpp17InputIterator requirements by being move-only, or by not being Cpp17EqualityComparable. That said, I think trying to list out properties case-by-case for each Cpp17 iterator category and each iterator adaptor that needs to decide its iterator category is a design error. We should use conccepts to decide whether an iterator meets the requirements of the various Cpp17 categories - ideally the exposition-only concepts from [iterator.traits].

[morinmorin]

A model of input_iterator may fail to meet the Cpp17InputIterator requirements by being move-only, or by not being Cpp17EqualityComparable.

That's right, ouch...

We should use conccepts to decide whether an iterator meets the requirements of the various Cpp17 categories - ideally the exposition-only concepts from [iterator.traits]

Agreed. Is that equivalent to not defining nested iterator_category in the adapted iterator (and then users query the iterator category through std::iterator_traits<…>::iterator_category)?

[morinmorin]

A model of input_iterator may fail to meet the Cpp17InputIterator requirements by being move-only, or by not being Cpp17EqualityComparable.

So, this means that the specification of iterator_category for many of range adaptor's iterators ([range.adaptors]) and iterator adaptors ([predef.iterators]) needs to be fixed.

[morinmorin]

This is now LWG 3301.