basic_waitable_timer& operator=(basic_waitable_timer&& rhs);
Effects: Cancels any outstanding asynchronous operations associated with *this as if by calling cancel(), then moves into *this the state originally represented by rhs.
Postconditions:
get_executor() == rhs.get_executor().
expiry() returns the same value as rhs.expiry() prior to the assignment.
rhs.expiry() == time_point().
Returns: *this.