Proposed resolution:

This wording is relative to N4988.

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

   constexpr iterator(iterator<!Const> i)
       requires Const && convertible_to<iterator_t<V>, OuterIter> &&
                convertible_to<iterator_t<InnerRng>, InnerIter> &&
                convertible_to<iterator_t<Pattern>, PatternIter>;
   

   -?- Preconditions: inner_it_.valueless_by_exception() is false.

   -10- Effects: […]

   constexpr decltype(auto) operator*() const;
   

   -?- Preconditions: inner_it_.valueless_by_exception() is false.

   -12- Effects: Equivalent to: […]

   constexpr iterator& operator++();
   

   -?- Preconditions: inner_it_.valueless_by_exception() is false.

   -13- Effects: Equivalent to: […]

   constexpr void operator++(int);
   

   -?- Preconditions: inner_it_.valueless_by_exception() is false.

   -14- Effects: Equivalent to: ++*this.

   constexpr iterator operator++(int)
     requires ref-is-glvalue && forward_iterator<OuterIter> && forward_iterator<InnerIter>;
   

   -?- Preconditions: inner_it_.valueless_by_exception() is false.

   -15- Effects: Equivalent to: […]

   constexpr iterator& operator--()
     requires ref-is-glvalue && bidirectional_range<Base> &&
              bidirectional-common<InnerBase> && bidirectional-common<PatternBase>;
   

   -?- Preconditions: inner_it_.valueless_by_exception() is false.

   -16- Effects: Equivalent to: […]

[ 2024-08-08, Inbal Levi provides wording ]

[ 2024-06-24; Reflector poll ]

Set priority to 3 after reflector poll. We like the suggestion to require "not valueless" as a precondition.

join_with_view's iterator is defined in terms of variant, visit and get, which implies that the iterator operations throw bad_variant_access if the underlying variant is valueless-by-exception (which can happen if an underlying iterator has a throwing copy constructor).

IMO, the variant is an implementation detail and shouldn't be exposed. It's confusing for users to get bad_variant_access when user code does not deal with variant.

The spec of join_with_view is also inconsistent with concat_view as recently added to the working draft by P2542R8. The latter has "it_.valueless_by_exception() is false" as a precondition.

I believe that join_with_view should similarly require that inner_it_.valueless_by_exception() is false for each iterator operation.

(FWIW, MSVC STL implements join_with_view with a trimmed-down version of variant, and having to throw bad_variant_access causes a small maintenance burden.)