3433. subrange::advance(n) has UB when n < 0

Section: 25.5.4.3 [range.subrange.access] Status: C++23 Submitter: Casey Carter Opened: 2020-04-21 Last modified: 2023-11-22

Priority: 2

View all other issues in [range.subrange.access].

View all issues with C++23 status.

Discussion:

subrange::advance(n) is specified to call ranges::advance(begin_, n, end_) in both StoreSize and !StoreSize cases (25.5.4.3 [range.subrange.access]/9). Unfortunately, ranges::advance(begin_, n, end_) has undefined behavior when n < 0 unless [end_, begin_) denotes a valid range (24.4.4.2 [range.iter.op.advance]/5). This would all be perfectly fine — the UB is exposed to the caller via effects-equivalent-to — were it not the design intent that subrange::advance(-n) be usable to reverse the effects of subrange::advance(n) when the subrange has a bidirectional iterator type. That is, however, clearly the design intent: subrange::prev(), for example, is equivalent to subrange::advance(-1).

[2020-05-11; Reflector prioritization]

Set priority to 2 after reflector discussions.

[2021-01-15; Issue processing telecon]

Set status to Tentatively Ready after discussion and poll.

FAN
703

[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

  1. Modify 25.5.4.3 [range.subrange.access] as indicated:

    constexpr subrange& advance(iter_difference_t<I> n);

    -9- Effects: Equivalent to:

    1. (9.1) — If StoreSize is true,

      auto d = n - ranges::advance(begin_, n, end_);
if (d >= 0)
  size_ -= to-unsigned-like(d);
else
  size_ += to-unsigned-like(-d);
return *this;

    2. (9.2) — Otherwise,

      ranges::advance(begin_, n, end_);
return *this;

    if constexpr (bidirectional_iterator<I>) {
  if (n < 0) {
    ranges::advance(begin_, n);
    if constexpr (StoreSize)
      size_ += to-unsigned-like(-n);
    return *this;
  }
}

auto d = n - ranges::advance(begin_, n, end_);
if constexpr (StoreSize)
  size_ -= to-unsigned-like(d);
return *this;