Proposed resolution:

This wording is relative to N5046.

1. Modify [view.interface], class template `view_interface` synopsis, as indicated:

   namespace std::ranges {
     template<class D>
       requires is_class_v<D> && same_as<D, remove_cv_t<D>>
     class view_interface {
       […]
     public:
       […]
       template<random_access_range R = D>
         constexpr decltype(auto) operator[](range_difference_t<R> n) {
           return ranges::begin(derived())[n];
         };
       template<random_access_range R = const D>
         constexpr decltype(auto) operator[](range_difference_t<R> n) const {
           return ranges::begin(derived())[n];
         };
     };
   }
   

2. Modify [view.interface.members] as indicated:

   constexpr decltype(auto) back() requires bidirectional_range<D> && common_range<D>;
   constexpr decltype(auto) back() const
     requires bidirectional_range<const D> && common_range<const D>;
   

   -3- Hardened preconditions: `!empty()` is `true`.

   -4- Effects: Equivalent to: return *ranges::prev(ranges::end(derived()));

   
   template<random_access_range R = D>
     constexpr decltype(auto) operator[](range_difference_t<R> n);
   template<random_access_range R = const D>
     constexpr decltype(auto) operator[](range_difference_t<R> n) const;
   

   -?- Hardened preconditions: If `D` models `sized_range`, then n >= 0 && n < ranges::distance(derived()) is `true`.

   -?- Effects: Equivalent to: return ranges::begin(derived())[n];

Hardening `view_interface::operator[]` to prevent out-of-bounds access is worthwhile for the `sized_range` cases, and we have already done so for other standard views such as `span` and `string_view`'s `operator[]`.