    Construct from nullptr.
                </p>
                <p>
                  Copy constructor.
                </p>
                <p>
                  Move constructor.
                </p>
                <p>
                  Construct a polymorphic wrapper for the specified executor.
                </p>
                <p>
                  Allocator-aware constructor to create a polymorphic wrapper for
                  the specified executor.
                </p>
              </td>
</tr>