bug#66187: read-file-name unexpected behavior when MUSTMATCH is a function

[Joseph Turner via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>>> For example, given:
>>>
>>> (completing-read "Prompt: " '("a" "b") nil
>>>                  (lambda (input)
>>>                    (string= "a" input)))
>>>
>>> I expected that the prompt would refuse to complete "b". I was wrong.
>>>
>>> Since completing-read is such a fundamental part of Emacs, I doubt it
>>> will be possible to change this behavior.  What do you think about the
>>> attached patch to clarify the completing-read docstring?
>
> The use of a function as `require-match` is brand new in Emacs-29, so
> I think it's not too late to fix it.  I think rather than fixing the doc
> we should fix the behavior, e.g. with the patch below.
>
>
>         Stefan
>
>
> diff --git a/lisp/minibuffer.el b/lisp/minibuffer.el
> index 2120e31775e..d4da2d0d19b 100644
> --- a/lisp/minibuffer.el
> +++ b/lisp/minibuffer.el
> @@ -1835,15 +1835,13 @@ completion--complete-and-exit
>    (cond
>     ;; Allow user to specify null string
>     ((= beg end) (funcall exit-function))
> -   ;; The CONFIRM argument is a predicate.
> -   ((and (functionp minibuffer-completion-confirm)
> -         (funcall minibuffer-completion-confirm
> -                  (buffer-substring beg end)))
> -    (funcall exit-function))
>     ;; See if we have a completion from the table.
> -   ((test-completion (buffer-substring beg end)
> -                     minibuffer-completion-table
> -                     minibuffer-completion-predicate)
> +   ((if (functionp minibuffer-completion-confirm)
> +        (funcall minibuffer-completion-confirm
> +                 (buffer-substring beg end))
> +      (test-completion (buffer-substring beg end)
> +                       minibuffer-completion-table
> +                       minibuffer-completion-predicate))
>      ;; FIXME: completion-ignore-case has various slightly
>      ;; incompatible meanings.  E.g. it can reflect whether the user
>      ;; wants completion to pay attention to case, or whether the

Thank you!! The above change fixes completion--complete-and-exit:
attempting to exit with "b" now runs the final catchall cond clause.

However, the unexpected behavior persists (it is possible to exit with
"b") because the completion-function that completion-complete-and-exit
passes to completion--complete-and-exit returns "b" anyway since "b"
exactly matches one of elements of COLLECTION.

Thank you!

Joseph