Proposed resolution:

This wording is relative to N4917.

1. Modify [ranges.cartesian.iterator] as indicated:

   namespace std::ranges {
     template<input_range First, forward_range... Vs>
       requires (view<First> && ... && view<Vs>)
     template<bool Const>
     class cartesian_product_view<First, Vs...>::iterator {
     public:
       […]
       iterator() requires forward_range<maybe-const<Const, First>> = default;
       […]
     private:
       using Parent = maybe-const<Const, cartesian_product_view>;          // exposition only
       Parent* parent_ = nullptr;                                          // exposition only
       tuple<iterator_t<maybe-const<Const, First>>,
         iterator_t<maybe-const<Const, Vs>>...> current_;                  // exposition only
       […]
     };
   }
   

Currently, cartesian_product_view::iterator only provides the default constructor when the first range models forward_range, which seems too restrictive since several input iterators like istream_iterator are still default-constructible.

It would be more appropriate to constrain the default constructor only by whether the underlying iterator satisfies default_initializable, as most other range adaptors do. Since cartesian_product_view::iterator contains a tuple member that already has a constrained default constructor, the proposed resolution simply removes the constraint.