Issue 3115: Unclear description for algorithm includes

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3115. Unclear description for algorithm `includes`

Section: 26.8.7.2 [includes] Status: Resolved Submitter: Casey Carter Opened: 2018-05-24 Last modified: 2020-09-06

Priority: 3

View all other issues in [includes].

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Discussion:

26.8.7.2 [includes]/1 states:

Returns: true if [first2, last2) is empty or if every element in the range [first2, last2) is contained in the range [first1, last1). Returns false otherwise.

but this program:

#include <algorithm>
#include <array>

int main() {
  std::array<int, 1> a{1};
  std::array<int, 3> b{1,1,1};
  return std::includes(a.begin(), a.end(), b.begin(), b.end());
}

returns 0 on every implementation I can find, despite that every element in the range b is contained in the range a. The design intent of the algorithm is actually to determine if the sorted intersection of the elements from the two ranges — as would be computed by the set_intersection algorithm — is the same sequence as the range [first2, last2). The specification should say so.

The complexity bound in 26.8.7.2 [includes]/2 is also unnecessarily high: straightforward implementations perform at most 2 * (last1 - first1) comparisons.

Previous resolution [SUPERSEDED]:

This wording is relative to N4750.
Modify 26.8.7.2 [includes] as indicated:
template<class InputIterator1, class InputIterator2>
  constexpr bool includes(InputIterator1 first1, InputIterator1 last1,
                          InputIterator2 first2, InputIterator2 last2);
[…]
-1- Returns: true if and only if [first2, last2) is ~~empty or if every element in the range [first2, last2) is contained in the range [first1, last1). Returns false otherwise~~a subsequence of [first1, last1).
-2- Complexity: At most 2 * ((last1 - first1) + (last2 - first2)) - 1 comparisons.

[2018-06-27 after reflector discussion]

Priority set to 3. Improved wording as result of that discussion.

[2018-11-13; Casey Carter comments]

The acceptance of P0896R4 during the San Diego meeting resolves this issue: The wording in [includes] includes the PR for LWG 3115.

Proposed resolution:

This wording is relative to N4750.

Modify 26.8.7.2 [includes] as indicated:
```
template<class InputIterator1, class InputIterator2>
  constexpr bool includes(InputIterator1 first1, InputIterator1 last1,
                          InputIterator2 first2, InputIterator2 last2);
[…]
```
-1- Returns: true if and only if [first2, last2) is ~~empty or if every element in the range [first2, last2) is contained in the range [first1, last1). Returns false otherwise~~a subsequence of [first1, last1). [Note: A sequence S is a subsequence of another sequence T if S can be obtained from T by removing some, all, or none of T's elements and keeping the remaining elements in the same order. — end note].
-2- Complexity: At most 2 * ((last1 - first1) + (last2 - first2)) - 1 comparisons.

3115. Unclear description for algorithm includes

3115. Unclear description for algorithm `includes`