bug#63744: 28.2; fix dired-guess-default

[Eli Zaretskii <eliz@gnu.org>]

> From: Michael Heerdegen <michael_heerdegen@web.de>
> Cc: Leo Liu <sdl.web@gmail.com>,  63744@debbugs.gnu.org
> Date: Tue, 30 May 2023 02:24:25 +0200
> 
> then you meant to recompute the variable's value every time the user wants
> to open a file with dired?

No, every time the situation changes in a way that requires its
recomputation.  I don't have a clear idea when that happens, but if
the REGEXP part is ".", it is clear to me it happens quite often
already, since the list of commands for a given file depends on the
file, and "." does not.

> It's a defcustom so Emacs should not change the value at all.

We are talking about user customizations.  A user can make his/her
customizations change the value as often as needed.

> And recomputing the whole value every time is much slower than computing
> a list of defaults for one class of files matched by the regexp.

That is not Leo's use case, AFAICT: the list of commands is
dynamically recomputed when needed anyway.

> The conditions that need to be checked are very different for
> different file types in a typical scenario, and the suggested design
> is more efficient: first look at the file type by matching the RE
> part, then decide, at the moment of user request, what applications
> are appropriate for this type of file (and only for this type) in
> this moment.

Like I said: I'm not against extending the value to support this, I
just don't see how this could be considered an urgent bugfix that must
be done in Emacs 29, that's all.