This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of New status.

4624. Inconsistent preconditions in repeat_view::iterator

Section: 25.6.5.3 [range.repeat.iterator] Status: New Submitter: Tomasz Kamiński Opened: 2026-09-21 Last modified: 2026-09-21

Priority: Not Prioritized

View other active issues in [range.repeat.iterator].

View all other issues in [range.repeat.iterator].

View all issues with New status.

Discussion:

As currently specified, the preconditions requiring current_ to be non-negative (25.6.5.3 [range.repeat.iterator] p2, p6, p8, p10) are applied only if "If Bound is not unreachable_sentinel_t". In consequence, decrementing iterator before begin is UB only if views::repeat specifies bound: 

auto rv1 = views::repeat(1, 10);
auto it1 = std::prev(rv1.begin()); // UB
auto rv2 = views::repeat(1);
auto it2 = std::prev(rv2.begin()); // OK
In contrast to the precondition on bound in repeat_view constructor, the current_ is of ptrdiff_t type in such case, and the check can always be performed.

On the LWG reflector Michał Dominiak (the author of the paper) confirmed, that this is unintentional copy-paste error in the wording, and the precondition should always be applied.

Proposed resolution:

This wording is relative to N5054.

The proposed resolution completely removes the precondition on private constructor iterator(const T* value, index-type b = index-type()), as it is private and can only be invoked by the library internally.

  1. Modify 25.6.5.3 [range.repeat.iterator] as indicated:

    constexpr explicit iterator(const T* value, index-type b = index-type());

    -1- Preconditions: If Bound is not unreachable_sentinel_t, b ≥ 0.

    -2- Effects: […]

    […] 
    constexpr explicit iterator& operator--();

    -6- Preconditions: If Bound is not unreachable_sentinel_t, current_ ≥ 0.

    -7- Effects: […]

    constexpr explicit iterator& operator+=(difference_type n);

    -8- Preconditions: If Bound is not unreachable_sentinel_t, current_ + n ≥ 0.

    -9- Effects: […]

    constexpr explicit iterator& operator-=(difference_type n);

    -10- Preconditions: If Bound is not unreachable_sentinel_t, current_ - n ≥ 0.

    -11- Effects: […]