Transfer ownership of the asynchronous result associated with one
object to another.
packaged_task& operator=(packaged_task&& other);
Transfer ownership of the asynchronous result and task associated
already had an associated asynchronous result, that asynchronous
result is made ready with an exception of
std::future_error, and an error code
The asynchronous result and task associated with
other prior to the invocation
of the move-assignment operator is associated with the
other has no associated
asynchronous result or task.