Hi Drew,

I'm sorry it took me so long to actually take a look at this.  I had to
reread the conversation to put myself back into topic.

As I said in https://debbugs.gnu.org/cgi/bugreport.cgi?bug=59937#26
I was looking for a way to:
1. Being able to create the restricted-sexp widget, even if the default
value for it isn't valid.
2. Being able to do it without prompting the user.

That's what the attached patch does.  AFAIU, we both agreed that that
approach was a good enough fix for this bug, so that the behavior of
certain defcustoms is less confusing.  We didn't really agree back then
about the warning, and I'm guessing that we still disagree on that, so I
didn't change anything about the warning.