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<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;
      }
      

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

[ 2021-06-23; Reflector poll ]

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

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.