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

[Joseph Turner]

Michael Heerdegen <michael_heerdegen@web.de> writes:

> AFAIU the algorithm
> works like this: first it's checked whether a completion is allowed (by
> checking whether it's the name of an existing file and consulting
> PREDICATE when specified).  Only when it is not allowed we check whether
> MUSTMATCH declares it acceptable.

Thank you!! That makes sense now.

> This is not really explained in the docstring, however.
>
> In your example, the default-directory is the name of an existing file,
> so the MUSTMATCH argument is irrelevant, the specified PREDICATE is
> fulfilled, so it's accepted as input.
>
> If I guess correctly I think you want something like this:
>
> #+begin_src emacs-lisp
> (let ((acceptable-p (lambda (filename)
>                       (and (file-directory-p filename)
>                            (directory-empty-p filename)))))
>   (read-file-name "Clone into new or empty directory: " nil nil
>                   (lambda (filename)
>                     (if (file-exists-p filename)
>                         (funcall acceptable-p filename)
>                       t))
>                   nil
>                   acceptable-p))
> #+end_src
>
> Does that come close?

Thank you! What I was hoping for may not have a clean solution:

- the completions buffer displays and allows tab-completion for all
  directories, empty or not, then
- upon pressing RET on a non-empty directory, the "[Match required]"
  message appears. Only upon pressing RET on an empty directory does
  completion succeed.

If this cannot be accomplished elegantly, I'll consider other designs
for the interactive prompt in package-vc-checkout.

Thanks!!

Joseph