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

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

> Coming back to this, perhaps a good place to warn about a bad default value is where we find it for the first time.  So I attach a patch that makes the restricted-sexp widget warn (but not error out) if the internal value is not a string.

> When the user clicks the INS button, the following warning pops up:
> Warning (widget-bad-default-value): 
> A widget of type restricted-sexp has a bad default value.
> value: nil
> match function: widget-restricted-sexp-match
> match-alternatives: (functionp)
>
> which I hope conveys good enough information to fix the mistake.
>
> I made it just a warning, because this mistake doesn't always result in
> a messed up buffer.  But it can be changed to an error, of course.

I may have forgotten some of what this is about.  If so,
please ignore...

If the problem is the default value then it's not up to
a user to fix it, and most users won't know how to deal
with such a warning (or error).  They can expect warnings
and errors about their own behavior, but not messages
about some problem with the defcustom definition.

If the problem can't be detected before a user tries to
customize, then maybe, when she does, the warning should
make it very clear that the _default_ value is a mismatch
and advise the user to report a bug to the library author.

IOW make clear that it's not about the user doing
something wrong (and don't prevent the user from
continuing to customize to a valid value).  Make it very
clear that the problem is with the maintainer of the code,
and suggest that the user report the problem.  And give
the user some detailed info that can be copied in a report
to the library maintainer.

Does this make sense, or am I missing something?