bug#17234: 24.3.50; overlay priority : cons cells make an error in ediff

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: rgm@gnu.org,  17234@debbugs.gnu.org
> Date: Fri, 11 Apr 2014 11:56:24 -0400
> 
> > Then how can we expect authors of other Lisp packages to be able to
> > fix their code so it works with Emacs 24.4 and later?  We must say
> > _something_ in NEWS.
> 
> It has always been the case that `priority' could be any value.
> 
> Any non-number value was treated as nil by the display engine, whereas
> now some cons values are treated as something else.
> 
> But ediff's bug could already be triggered in Emacs<24.4 by a package
> installing an overlay with a `priority' that's a cons cell.

Sorry, I don't see how all this removes the need to inform Lisp
programmers of the change of a long-time traditional behavior.

I don't understand the reason(s) for hiding this information.  If you
don't want to divulge some internal details, the information can be
conveyed in a purely functional manner ("don't do XXX; instead, do
YYY" or "don't assume that overlay priority is either an integer or
nil) that doesn't require to describe implementation details.  But I
don't see how can we say nothing about this incompatible change.