Proposed resolution:

This wording is relative to the post-2021-February-virtual-meeting working draft.

1. Modify [range.join.iterator] as indicated:

   -1- iterator::iterator_concept is defined as follows:

   1. (1.1) — If ref-is-glvalue is true, and Base and range_reference_t<Base> each models bidirectional_range, and range_reference_t<Base> models both bidirectional_range and common_range, then iterator_concept denotes bidirectional_iterator_tag.

   2. […]

   […]

   -2- The member typedef-name iterator_category is defined if and only if ref-is-glvalue is true, Base models forward_range, and range_reference_t<Base> models forward_range. In that case, iterator::iterator_category is defined as follows:

   1. (2.1) — Let OUTERC denote iterator_traits<iterator_t<Base>>::iterator_category, and let INNERC denote iterator_traits<iterator_t<range_reference_t<Base>>>::iterator_category.

   2. (2.2) — If OUTERC and INNERC each model derived_from<bidirectional_iterator_tag>, and range_reference_t<Base> models common_range, iterator_category denotes bidirectional_iterator_tag.

   3. […]

[ 2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP. ]

[ 2021-06-23; Reflector poll ]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[ 2021-04-20; Reflector poll ]

Priority set to 2.

[ 2021-04-04; Daniel comments ]

The below proposed wording can be compared as being based on N4885.

Per [range.join.iterator]/1, join_view::iterator::iterator_concept denotes bidirectional_iterator_tag if ref-is-glvalue is true and Base and range_reference_t<Base> each model bidirectional_range. Similarly, paragraph 2 says that join_view::iterator::iterator_category is present if ref-is-glvalue is true, and denotes bidirectional_iterator_tag if the categories of the iterators of both Base and range_reference_t<Base> derive from bidirectional_iterator_tag.

However, the constraints on the declarations of operator-- and operator--(int) in the synopsis that immediately precedes paragraph 1 disagree. Certainly they also consistently require ref-is-glvalue && bidirectional_range<Base> && bidirectional_range<range_reference_t<Base>>, but they additionally require common_range<range_reference_t<Base>>. So as currently specified, this iterator sometimes declares itself to be bidirectional despite not implementing --. This is not incorrect for iterator_concept — recall that iterator_concept is effectively an upper bound since the concepts require substantial syntax — but slightly misleading. It is, however, very much incorrect for iterator_category which must not denote a type derived from a tag that corresponds to a stronger category than that iterator implements.

It's worth pointing out, that LWG 3313 fixed the constraints on operator--() and operator--(int) by adding the common_range requirements, but failed to make a consistent change to the definitions of iterator_concept and iterator_category.