bug#36516: Wrong dynamic abbrev expansion after space

[Alan Third <alan@idiocy.org>]

On Wed, Jul 24, 2019 at 12:15:16AM +0300, Juri Linkov wrote:
> >> 2. On the third line type:
> >>  s                      ;; self-insert-command
> >>  M-/                    ;; dabbrev-expand
> >>  SPC                    ;; self-insert-command
> >>  M-/                    ;; dabbrev-expand
> >>  M-/                    ;; dabbrev-expand
> >> 3. The word after "str " is expanded to some random word.
> >
> > It's due to this code in dabbrev.el:
> >
> > (defun dabbrev-expand (arg)
> >    ...
> >         ;; If the user inserts a space after expanding
> >         ;; and then asks to expand again, always fetch the next word.
> >         (if (and (eq (preceding-char) ?\s)
> >                  (markerp dabbrev--last-abbrev-location)
> >                  (marker-position dabbrev--last-abbrev-location)
> >                  (= (point) (1+ dabbrev--last-abbrev-location)))
> >             (progn
> >               ;; The "abbrev" to expand is just the space.
> >               (setq abbrev " ")
> >               ...
> >
> > This triggers on the second M-/ in your recipe above, which causes the
> > third M-/ to expand " " instead of "str ".  Not sure what to do about it
> > though.  Removing this `if' will fix your case, but probably break
> > others.
> 
> Thanks for investigating this.  Out of curiosity, I checked now
> the test directory, and here it is: test/lisp/dabbrev-tests.el -
> the already existing test with exactly the same recipe as in my report.
> But for some reason this test doesn't fail.

I remembered writing that test and checked previously. It’s not
exactly the same recipe. That one was where there were two spaces and
subsequent calls to dabbrev-expand would replace the wrong text with
the correct expansion, not replace the correct text with the wrong
expansion.

I hope that makes sense... A bit more info here:

    https://lists.gnu.org/archive/html/emacs-devel/2016-01/msg01721.html
-- 
Alan Third