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

From: Michael Heerdegen <[email-masked]>
Date: Wed, 27 Sep 2023 02:26:20 +0200

Joseph Turner <[email-masked]> writes:

> 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.

Ah - ok, now finally I've understood all the parts.

You want to prompt for a directory that is either empty or not yet
existing.  But with `read-file-name' you only get either (a) non-empty
directories accepted as input, or (b) failing completion of existing
non-empty directories, which makes it impossible to choose a directory
inside an existing directory.

Yes, looks like a bug that this is not possible.  It should be possible
to complete directory names that do not match.

Then the documentation needs to be improved: what exactly is accepted
when both MUSTMATCH and PREDICATE are specified?

Finally, I think the docstring of `read-directory-name' needs to be
updated: it fails to mention that MUSTMATCH can be a function (the
argument is passed directly to `read-file-name').

Michael.