Proposed resolution:

Resolved by accepting P1456R1.

[ 2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved. ]

[ 2019-12-16; Casey comments ]

This issue has been resolved by P1456R1 "Move-only views", which added no less than two member functions named "base" to join_view.

Previous resolution [SUPERSEDED]:

This wording is relative to N4835.

1. Modify [range.join.view], class template join_view synopsis, as indicated:

   […]
   template<input_range R>
     requires viewable_range<R> && constructible_from<V, all_view<R>>
   constexpr explicit join_view(R&& r);
   
   constexpr V base() const;
   
   constexpr auto begin() {
     return iterator<simple-view<V>>{*this, ranges::begin(base_)};
   }
   […]
   

2. Modify [range.join.view] as indicated:

   template<input_range R>
     requires viewable_range<R> && constructible_from<V, all_view<R>>
   constexpr explicit join_view(R&& r);
   

   -2- Effects: […]

   constexpr V base() const;
   

   -?- Effects: Equivalent to: return base_;

[ 2019-11 Status to Ready during Wednesday night issue processing in Belfast. ]

Addresses US 293

join_view is missing a base() member for returning the underlying view. All the other range adaptors provide this.

Proposed change:

To the join_view class template add the member:

constexpr V base() const { return base_; }

Jonathan Wakely:

The NB comment says "join_view is missing a base() member for returning the underlying view. All the other range adaptors provide this."

In fact, split_view and istream_view do not provide base() either. Of the views that do define base(), all except all_view do so out-of-line, so the proposed resolution adds it out-of-line too.