bug#69501: 30.0.50; customize-mode error with substring completion style

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 5 Mar 2024 09:36:01 -0300
> Cc: 69501@debbugs.gnu.org
> From: Mauro Aranda <maurooaranda@gmail.com>
> 
> On 4/3/24 10:16, Eli Zaretskii wrote:
>  >> Date: Sat, 02 Mar 2024 09:59:48 +0100
>  >> From:  Eshel Yaron via "Bug reports for GNU Emacs,
>  >>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
>  >>
>  >> With both Emacs 29.2 and with the current master:
>  >>
>  >> 1. emacs -Q
>  >> 2. (setq completion-styles '(substring))
>  >> 3. C-u M-x customize-mode TAB
>  >> 4. "Assertion failed: (stringp suffix)"
>  >>
>  >> I not sure exactly why this error occurs, but IIUC it has to do with the
>  >> fact that customize-mode let-binds completion-regexp-list.  I suspect
>  >> that the substring completion style doesn't handle that correctly.
>  >>
>  >> In general, let-binding completion-regexp-list around completing-read is
>  >> not a great idea IMO because it affects recursive minibuffers as well.
>  >> So perhaps the best solution is to avoid doing that in customize-mode,
>  >> and to rely only on the predicate argument of completing-read instead?
>  >
>  > Stefan and Mauro, any suggestions?
> 
> I took a quick look, but I don't know how "relying on the predicate
> argument of completing-read" looks in practice.  It seems I have
> forgotten how to use the completion functions (if I ever knew how to use
> them at all).  Hopefully Stefan will be of more help.

Stefan?