Proposed resolution:

This wording is relative to N4830.

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

   [Drafting note: Changing the join_view<V>::inner_ member to be mutable is safe because this exposition-only member is only used when the join_view is single-pass and only modified by operations that invalidate other iterators]

   namespace std::ranges {
     template<input_range V>
       requires view<V> && input_range<range_reference_t<V>>> &&
               (is_reference_v<range_reference_t<V>> ||
               view<range_value_t<V>>)
     class join_view : public view_interface<join_view<V>> {
     private:
       […]
       V base_ = V(); // exposition only
       mutable all_view<InnerRng> inner_ = // exposition only, present only when 
                                           // !is_reference_v<InnerRng>
         all_view<InnerRng>();
     public:
       […]
     };
   }
   

[ 2020-08-21 Issue processing telecon: resolved by P1983R0 §2.4. Status changed: New → Resolved. ]

[ 2020-02-10, Prague ]

Would be resolved by P1983R0.

[ 2019-10 Priority set to 2 after reflector discussion ]

The non-const join_view::begin() returns iterator<simple-view<V>>. If simple-view<V> is true, then the iterator stores a const join_view* named parent_. iterator::satisfy() will try to write to parent_->inner_ if ref_is_glvalue is false. That doesn't work because the inner_ field is not marked mutable.