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

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 01 Oct 2011 00:33:49 -0700
> From: Paul Eggert <eggert@cs.ucla.edu>
> Cc: 9642@debbugs.gnu.org
> 
> On 09/30/11 21:01, Stefan Monnier wrote:
> 
> > if the user goes through the trouble to do the above, he probably
> > doesn't want this overlay to disappear right away
> 
> I don't see why not.  Evaporating overlays are supposed
> disappear when they become empty -- that's their whole point.
> Why would one want an evaporating overflow that's empty?

But what is the meaning of moving an overlay to begin at buffer
position zero?  Such a position doesn't exist in any buffer.  So doing
this invokes undefined behavior, whose result can be anything,
including the end of the world.

> Let's look at it a different way.  Suppose we were to try
> to document the current behavior.  We'd add something to
> the manual like "overlays with the evaporate property are deleted when
> they become empty, except when X".  What would X be,

X == "empty means the beginning and the end positions of the overlay
are identical".