bug#25152: 25.1; Customize: errors for `restricted-sexp' in `repeat'

[Drew Adams <drew.adams@oracle.com>]

>> emacs -Q
>>
>> (defcustom bar
>>   `(ignore)
>>   "..."
>>   :type
>>   '(repeat (restricted-sexp :match-alternatives (functionp)))
>>   :group 'emacs)
>>
>> M-x customize-option bar
>>
>> 1. Click the INS button, to insert a new element in the list.
>> 2. At the prompt "Lisp expression: ", hit `C-g'.
>
> You shouldn't be prompted, because the widget library is not ready to
> take user input at this stage (the stage being the creation of the
> widget).
>
> The prompt comes from the function :value-to-external of the sexp
> widget, which is called in the process of creating the widget.  The
>:value-to-external function calls read with the value of the widget,
> like this: (read value)
>
> It does that assuming value is a string.  It does not want user input
> (by reading it from the minibuffer), it just wants to take the widget
> value and return it in the "external" format.  The function
>:value-to-internal is the one that should print the value of the widget
> to a string.  But it only does that if the value matches the widget, as
> with the function :match, which in the defcustom posted in the recipe is
> widget-restricted-sexp-match.
>
> So, why doesn't the value of the widget satisfy
> widget-restricted-sexp-match, at the widget creation stage?  That's
> because :match-alternatives is '(functionp), and the default value of
> the widget is nil.  Because of that, widget-restricted-sexp-match
> returns nil, and we store the value intact (i.e., not in the "internal"
> format), and we end up evaluating (read nil), resulting in the prompt
> and losing badly.
>
> But, the Elisp manual says:
> ‘:value DEFAULT’
>     Provide a default value.
>
>     If ‘nil’ is not a valid value for the alternative, then it is
>     essential to specify a valid default with ‘:value’.
>
> So the defcustom provided is lacking the essential :value DEFAULT thing.
> If we compare the behavior with the following defcustom:
> (defcustom bar
>  `(ignore)
>  "..."
>  :type
>  '(repeat (restricted-sexp :match-alternatives (functionp)
>                            :value ignore))
>  :group 'emacs)
>
> And in the customization buffer we click the INS button, then there's no
> prompt: creation of the widget happens with no problem.
>
> Another defcustom, that makes nil a valid value:
> (defcustom bar
>  `(ignore)
>  "..."
>  :type
>  '(repeat (restricted-sexp :match-alternatives (functionp null)))
>  :group 'emacs) 
>
> Again, click the INS button: no problem.
>
>
> To sum it up, the prompt is an unfortunate mistake, and maybe we could
> protect against that, but I think the real problem comes from the
> defcustom, which fails to provide a valid default value.

Thanks for this explanation.  Makes sense.

Can we somehow help users by raising an error when they
do this?  Seems like a simple mistake to make.