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>]

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Michael Heerdegen <michael_heerdegen@web.de> writes:
> 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.

I'm happy to work on the bug, if others agree that this change is desired.

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

I can improve the docs too, once I have the answer that questions.

> 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').

How about the attached patch?

> Michael.


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From e0ea8501af4e0dfd9777879e44a07902f86d1a7e Mon Sep 17 00:00:00 2001
From: Joseph Turner <joseph@breatheoutbreathe.in>
Date: Tue, 26 Sep 2023 18:01:27 -0700
Subject: [PATCH] Fix MUSTMATCH description in read-directory-name docstring

* lisp/files.el (read-directory-name)
---
 lisp/files.el | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lisp/files.el b/lisp/files.el
index b72f141c0ee..1fe124848b9 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -817,7 +817,7 @@ non-empty string that was inserted by this function.
 If the user exits with an empty minibuffer, this function returns
 an empty string.  (This can happen only if the user erased the
 pre-inserted contents or if `insert-default-directory' is nil.)
-Fourth arg MUSTMATCH non-nil means require existing directory's name.
+Fourth arg MUSTMATCH is passed as-is to `read-file-name', which see.
  Non-nil and non-t means also require confirmation after completion.
 Fifth arg INITIAL specifies text to start with.
 DIR should be an absolute directory name.  It defaults to
-- 
2.41.0