bug#28390: 26.0.50; overlays-at actually sorts by increating priority

[Eli Zaretskii <eliz@gnu.org>]

> From: joaotavora@gmail.com (João Távora)
> Date: Thu, 07 Sep 2017 23:21:15 +0100
> 
> Hi Stefan, maintainters
> 
> I believe I found a rather easy glitch in the doc of overlays-at. It
> says if SORTED in non-nil it sorts overlays by "decreasing" priority,
> but actually the reverse is true.
> 
> Line 3234 of buffer.c seems to confirm this:
> 
>   /* Sort the overlays into the proper order: increasing priority.  */
> 
> and compare_overlays passed to qsort() as well:
> 
>     return s1->priority < s2->priority ? -1 : 1;
> 
> I also did this test:
> 
>    (progn
>      (mapc #'delete-overlay (overlays-in (point-min) (point-max)))
>      (dotimes (i 4) (overlay-put (make-overlay 20 30) 'priority i))
>      (mapcar (lambda (ov) (overlay-get ov 'priority)) (overlays-at 20 t)))
> 
> this returns (0 1 2 3)
> 
> This is a 3-year-old docbug, so I'm suspicious I might be missing
> something. Anyway here's a docpatch.

I think the doc string says what the implementation was supposed to
do, so we need to change the implementation instead.  If you look at
the changeset where the SORTED argument was introduced, you will see
that the old code sorted the list returned by overlays-at in
descending order of priorities, i.e. overlays with the largest
priority first.  It used 'sort' like this:

  (sort (mapcar #'overlay-properties (overlays-at p))
        (lambda (A B) (> (or (cadr (memq 'priority A)) 0)
                    (or (cadr (memq 'priority B)) 0)))))

The doc string of 'sort' says that its PREDICATE function should
return non-nil if the first element passed to PREDICATE should sort
_before_ the second.  And the predicate above uses '>'.

So I think we should rather reverse the list which overlays-at
returns.

Stefan?