Issue 3572: copyable-box should be fully constexpr (original) (raw)

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3572. _copyable-box_ should be fully constexpr

Section: 25.7.3 [range.move.wrap] Status: C++23 Submitter: Tim Song Opened: 2021-06-19 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [range.move.wrap].

View all issues with C++23 status.

Discussion:

P2231R1 made optional fully constexpr, but missed its cousin_semiregular-box_ (which was renamed to _copyable-box_ by P2325R3). Most operations of _copyable-box_ are already constexpr simply because _copyable-box_ is specified in terms of optional; the only missing ones are the assignment operators layered on top when the wrapped type isn't assignable itself.

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify [range.copy.wrap] as indicated:

   -1- Many types in this subclause are specified in terms of an exposition-only class template _copyable-box_._copyable-box_<T> behaves exactly like optional<T> with the following differences:

   1. (1.1) — […]
   2. (1.2) — […]
   3. (1.3) — If copyable<T> is not modeled, the copy assignment operator is equivalent to:

      constexpr copyable-box& operator=(const copyable-box& that)
      noexcept(is_nothrow_copy_constructible_v) {
      if (this != addressof(that)) {
      if (that) emplace(*that);
      else reset();
      }
      return *this;
      }

   4. (1.4) — If movable<T> is not modeled, the move assignment operator is equivalent to:

      constexpr copyable-box& operator=(copyable-box&& that)
      noexcept(is_nothrow_move_constructible_v) {
      if (this != addressof(that)) {
      if (that) emplace(std::move(*that));
      else reset();
      }
      return *this;
      }