I've meditated on this and I've changed my mind. A task that awaits
itself is so obviously not following the task protocol that it should
be shot on sight. No exceptions. (Only the task itself should ever set
the result or exception, and *only* by returning or raising. So the
only way a task waiting for itself could be unblocked is through
cancellation, and there are better ways to wait forever until
cancelled.)

IOW @1st1 I think it's okay to detect this and raise an exception.