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3726. reverse_iterator::operator-> is underconstrained for non-pointer iterators

Section: 25.5.1.6 [reverse.iter.elem] Status: NAD Submitter: Hewill Kang Opened: 2022-06-27 Last modified: 2022-11-30

Priority: Not Prioritized

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Discussion:

For non-pointer types, reverse_iterator::operator-> only requires the expression i.operator->() to be well-formed.

Since the return type of this function is explicitly specified as pointer, this will cause a hard error in the function body when the return type of i.operator->() cannot be converted to pointer.

We should add a return type constraint for this.

[2022-08-23; Reflector poll: NAD]

pointer is iterator_traits<Iterator>::pointer, which is required to name decltype(i.operator->()) (25.3.2.3 [iterator.traits]/1) so the postulated problem simply does not arise in valid code.

[2022-11-30 LWG telecon. Status changed: Tentatively NAD → NAD.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 25.5.1.6 [reverse.iter.elem] as indicated:

   constexpr pointer operator->() const
     requires (is_pointer_v<Iterator> ||
               requires(const Iterator i) { { i.operator->() } -> convertible_to<pointer>; });
   

   -2- Effects:

   1. (2.1) — If Iterator is a pointer type, equivalent to: return prev(current);

   2. (2.2) — Otherwise, equivalent to: return prev(current).operator->();