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### 3330. Include `<compare>` from most library headers

Section: 17.13.2 [coroutine.syn], 19.5.2 [system.error.syn], 22.2.1 [utility.syn], 22.4.2 [tuple.syn], 22.5.2 [optional.syn], 22.6.2 [variant.syn], 20.2.2 [memory.syn], 22.11.1 [type.index.synopsis], 23.4.2 [string.syn], 23.3.2 [string.view.synop], 24.3.2 [array.syn], 24.3.3 [deque.syn], 24.3.4 [forward.list.syn], 24.3.5 [list.syn], 24.3.6 [vector.syn], 24.4.2 [associative.map.syn], 24.4.3 [associative.set.syn], 24.5.2 [unord.map.syn], 24.5.3 [unord.set.syn], 24.6.2 [queue.syn], 24.6.3 [stack.syn], 25.2 [iterator.synopsis], 26.2 [ranges.syn], 29.2 [time.syn], 31.12.4 [fs.filesystem.syn], 32.3 [re.syn], 33.4.2 [thread.syn] Status: C++20 Submitter: United States Opened: 2019-11-07 Last modified: 2021-02-25

Priority: 0

View all other issues in [coroutine.syn].

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Discussion:

Addresses US 181

The spaceship `operator<=>` is typically not usable unless the library header `<compare>` is directly included by the user. Many standard library headers provide overloads for this operator. Worse, several standard classes have replaced their existing definition for comparison operators with a reliance on the spaceship operator, and existing code will break if the necessary header is not (transitively) included. In a manner similar to the mandated library headers transitively `#include`-ing `<initializer_list>` in C++11, these headers should mandate a transitive `#include <compare>`.

Proposed change:

Add:

`#include <compare>`

to the header synopsis for each of the following headers:

>     <array>
>     <chrono>
>     <coroutine>
>     <deque>
>     <forward_list>
>     <filesystem>
>     <iterator>
>     <list>
>     <map>
>     <memory>
>     <optional>
>     <queue>
>     <ranges>
>     <regex>
>     <set>
>     <stack>
>     <string>
>     <string_view>
>     <system_error>
>     <thread>
>     <tuple>
>     <type_index>
>     <unordered_map>
>     <unordered_set>
>     <utility>
>     <variant>
>     <vector>

[2019-11 Moved to Ready on Friday AM in Belfast]

Proposed resolution:

This wording is relative to N4835.

1. Add

   >     #include <compare>

   to the following header synopses:

   1. 17.13.2 [coroutine.syn]
   2. 19.5.2 [system.error.syn]
   3. 22.2.1 [utility.syn]
   4. 22.4.2 [tuple.syn]
   5. 22.5.2 [optional.syn]
   6. 22.6.2 [variant.syn]
   7. 20.2.2 [memory.syn]
   8. 22.11.1 [type.index.synopsis]
   9. 23.4.2 [string.syn]
   10. 23.3.2 [string.view.synop]
   11. 24.3.2 [array.syn]
   12. 24.3.3 [deque.syn]
   13. 24.3.4 [forward.list.syn]
   14. 24.3.5 [list.syn]
   15. 24.3.6 [vector.syn]
   16. 24.4.2 [associative.map.syn]
   17. 24.4.3 [associative.set.syn]
   18. 24.5.2 [unord.map.syn]
   19. 24.5.3 [unord.set.syn]
   20. 24.6.2 [queue.syn]
   21. 24.6.3 [stack.syn]
   22. 25.2 [iterator.synopsis]
   23. 26.2 [ranges.syn]
   24. 29.2 [time.syn]
   25. 31.12.4 [fs.filesystem.syn]
   26. 32.3 [re.syn]
   27. 33.4.2 [thread.syn]