Proposed resolution:

This wording is relative to N4993.

1. Modify [iterator.concept.contiguous] as indicated:

   -1- The `contiguous_iterator` concept provides a guarantee that the denoted elements are stored contiguously in memory.

   template<class I>
     concept contiguous_iterator =
       random_access_iterator<I> &&
       derived_from<ITER_CONCEPT(I), contiguous_iterator_tag> &&
       is_lvalue_reference_v<iter_reference_t<I>> &&
       same_as<iter_value_t<I>, remove_cvref_t<iter_reference_t<I>>> &&
       requires(const I& i) {
         { to_address(i) } -> same_as<add_pointer_t<iter_reference_t<I>>>;
       };
   

   -2- Let `a` and `b` be dereferenceable iterators and `c` be a non-dereferenceable iterator of type `I` such that `b` is reachable from `a` and `c` is reachable from `b`, and let `D` be iter_difference_t<I>. The type `I` models `contiguous_iterator` only if

   1. (2.1) — `to_address(a) == addressof(*a)`,

   2. (2.2) — `to_address(b) == to_address(a) + D(b - a)`,

   3. (2.3) — `to_address(c) == to_address(a) + D(c - a)`,

   4. (2.?) — `to_address(I{})` is well-defined,

   5. (2.4) — `ranges::iter_move(a)` has the same type, value category, and effects as `std::move(*a)`, and

   6. (2.5) — if `ranges::iter_swap(a, b)` is well-formed, it has effects equivalent to `ranges::swap(*a, *b)`.

[ Wrocław 2024-11-23; Status changed: Voting → WP. ]

[ 2024-11-13; Reflector poll ]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

The design intent of the `contiguous_iterator` concept is that iterators can be converted to pointers denoting the same sequence of elements. This enables a common range `[i, j)` or counted range `i + [0, n)` to be processed with extremely efficient low-level C or assembly code that operates on `[to_address(i), to_address(j))` (respectively `to_address(i) + [0, n)`).

A value-initialized iterator `I{}` can be used to denote the empty ranges `[I{}, I{})` and `I{} + [0, 0)`. While the existing semantic requirements of `contiguous_iterator` enable us to convert both dereferenceable and past-the-end iterators with `to_address`, converting ranges involving value-initialized iterators to pointer ranges additionally needs `to_address(I{})` to be well-defined. Note that `to_address` is already implicitly equality-preserving for `contiguous_iterator` arguments. Given this additional requirement `to_address(I{}) == to_address(I{})` and `to_address(I{}) == to_address(I{)) + 0` both hold, so the two types of empty ranges involving value-initialized iterators convert to empty pointer ranges as desired.