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3572. copyable-box should be fully constexpr

Section: 24.7.3 [range.copy.wrap] Status: WP Submitter: Tim Song Opened: 2021-06-19 Last modified: 2021-10-17

Priority: Not Prioritized

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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 24.7.3 [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<T>) {
        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<T>) {
        if (this != addressof(that)) {
          if (that) emplace(std::move(*that));
          else reset();
        return *this;