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

[Mauro Aranda]

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.