bug#74410: 31.0.50; completion-at-point doesn't work in custom buffers

[Juri Linkov <juri@linkov.net>]

>> Or to make the binding conditional with something like this:
>>
>> @@ -1079,7 +1079,10 @@ widget-global-map
>>  (defvar widget-field-keymap
>>    (let ((map (copy-keymap widget-keymap)))
>>      (define-key map "\C-k" #'widget-kill-line)
>> -    (define-key map "\M-\t" #'widget-complete)
>> +    (define-key map "\M-\t" `(menu-item "" 'widget-complete
>> +                              :filter ,(lambda (cmd)
>> +                                         (unless completion-at-point-functions
>> +                                           cmd))))
>>      (define-key map "\C-m" #'widget-field-activate)
>>      ;; Since the widget code uses a `field' property to identify fields,
>>      ;; ordinary beginning-of-line does the right thing.
>
> Cute 🙂
> I really like the fact that Emacs can do this, but we shouldn't abuse
> it, because it makes it more difficult for users to figure out
> what's going on.

Indeed, this was just an example.  But it would be nice to do the same
one way or another.  Maybe 'widget-complete' itself should check for
'completion-at-point-functions', or something like this.