bug#9642: move-overlay creates an empty overlay with the evaporate property

[Stefan Monnier]

>> In a nonempty buffer, the following:
>> (let ((o (make-overlay 1 2)))
>> (overlay-put o 'evaporate t)
>> (move-overlay o 0 1))

>> returns an empty overlay that has the evaporate property.
>> But this is not supposed to happen: when an overlay with that
>> property becomes empty, it's supposed to be deleted.

> OTOH if the user goes through the trouble to do the above, he probably
> doesn't want this overlay to disappear right away.  So I'm not sure we
> want to second guess the user here.

I see I was confused: I didn't realize that your example's use of
out-of-bounds position was important, and I thought your change also
affected (move-overlay OL A A).  So, I now think your patch is
perfectly fine, thank you.


        Stefan