Proposed resolution:

This wording is relative to N4971.

1. Modify [iterator.operations] as indicated:

   template<class InputIterator>
     constexpr typename iterator_traits<InputIterator>::difference_type
       distance(InputIterator first, InputIterator last);
   

   -4-Preconditions: last is reachable from first, or InputIterator meets the Cpp17RandomAccessIterator requirements and first is reachable from last. The return type can represent the result.

   -5- Effects: If InputIterator meets the Cpp17RandomAccessIterator requirements, returns (last - first); otherwise, increments first until last is reached and returns the number of increments.

[ 2024-03-12; Jonathan provides improved wording ]

[ 2024-03-12; Reflector poll ]

Set priority to 4 after reflector poll.

The proposed change is wrong, the new wording only associates with the second condition, but should also apply when "last is reachable from first".

std::distance for random access iterators is defined in terms of (last - first) ([iterator.operations] p5) for Cpp17RandomAccessIterators.

However, this subtraction is not guaranteed to be well-defined ([random.access.iterators], Table [tab:randomaccessiterator]):

Table 91: Cpp17RandomAccessIterator requirements (in addition to Cpp17BidirectionalIterator) [tab:randomaccessiterator]

Expression	Return type	Operational semantics	Assertion/note pre-/post-condition
[…]
b - a	difference_type	return n;	Preconditions: there exists a value n of type difference_type such that a + n == b. b == a + (b - a).

For example, pointer subtraction is undefined if the result isn't representable as std::ptrdiff_t, and user-defined types with random access iterators aren't required to have a difference which is always representable by difference_type.

std::distance(&a, &b) can't be well-defined when &b - &a is not, so std::distance is missing a precondition.