bug#28513: 25.1; ido insists on guessing the wrong directory

[Lars Ingebrigtsen <larsi@gnus.org>]

Dmitry Gutov <dgutov@yandex.ru> writes:

> On 12.12.2020 14:10, Lars Ingebrigtsen wrote:
>> I don't use ido-mode for files normally -- are there any ido users here
>> that do?  If so, does this seem like expected behaviour to you?
>
> Looks like a bug, yes.
>
> The scenario looks pretty specific, though, that's probably why it
> hasn't come up before.

I have a slightly simpler reproduction case:

emacs -Q -f ido-mode lisp/abbrev.el
C-x C-w RET C-f RET

This should write the file to calc/abbrev.el, but prompts for
overwriting.

Here's the backtrace with debug-on-quit:

Debugger entered--Lisp error: (quit)
  read-from-minibuffer("File ‘~/src/emacs/trunk/lisp/abbrev.el’ exists; ov.
  y-or-n-p("File ‘~/src/emacs/trunk/lisp/abbrev.el’ exists; ov...")
  write-file("~/src/emacs/trunk/lisp/abbrev.el" t)
  funcall-interactively(write-file "~/src/emacs/trunk/lisp/abbrev.el" t)
  call-interactively(write-file)
  ido-file-internal(write write-file nil "Write file: " nil nil ignore)
  ido-write-file()

I was momentarily puzzled about why that call-interactively to
write-file didn't re-prompt about the location, but:

       ((eq ido-exit 'fallback)
	;; Need to guard setting of default-directory here, since
	;; we don't want to change directory of current buffer.
	(let ((default-directory ido-current-directory)
	      (read-file-name-function nil))
	  (setq this-command (or ido-fallback fallback 'find-file))
	  (run-hook-with-args 'ido-before-fallback-functions this-command)
	  (call-interactively this-command)))

So hitting `C-f' makes ido go into `fallback' mode?  Yes!

    (define-key map "\C-f" 'ido-magic-forward-char)

(defun ido-magic-forward-char (arg)
  "Move forward in user input or perform magic action.
If no user input is present, or at end of input, perform magic actions:
C-x C-b ... C-f  switch to `ido-find-file'.
C-x C-f ... C-f  fallback to non-Ido `find-file'.
C-x C-d ... C-f  fallback to non-Ido brief `dired'.
C-x d ... C-f    fallback to non-Ido `dired'."
  (interactive "P")
  (cond
   ((or arg (not (eobp)))
    (forward-char (min (prefix-numeric-value arg)
		       (- (point-max) (point)))))
   ((memq ido-cur-item '(file dir))
    (ido-fallback-command))

So...  this is apparently a feature?  Hitting `C-f' disables ido and
calls the fallback command, which is `write-region' in this case.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no