bug#56532: 27.2; Ido C-f in read-file-name inserts twice the initial name

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: Tobias Bora <tobias.bora@gmail.com>,  56532@debbugs.gnu.org
> Date: Thu, 14 Jul 2022 20:19:41 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> I can easily reproduce it in emacs -Q yes by simply evaluating the 
> >> following config:
> >> 
> >> (setq ido-everywhere t)
> >> (ido-mode)
> >> ;; Stop changing directory when openning a file that does not exist!
> >> (setq ido-auto-merge-work-directories-length -1)
> >> 
> >> (read-file-name  "Picture name:" "/tmp/" nil nil "defaultname.png")
> 
> Thanks; with that I can reproduce the problem.
> 
> > It looks like you aren't supposed to do that under Ido.  See
> > ido-magic-forward-char.
> 
> It may not be supposed to work, but it'd be logical if it did, I think?
> But I'm not that familiar with how ido does fallbacks (it seems to
> involve some strange, brittle magic), so I don't know whether it's
> possible or not.

I think it relies on how the candidates are displayed at that point,
which is quite different from what read-file-name does.  Yes, that's
brittle.