bug#33294: xwidget-insert crashes Emacs

[Eli Zaretskii]

> From: Stefan Monnier <monnier@IRO.UMontreal.CA>
> Cc: Robert Pluim <rpluim@gmail.com>, lg.zevlg@gmail.com, gazally@runbox.com,
>         33294@debbugs.gnu.org
> Date: Thu, 08 Nov 2018 15:15:50 -0500
> 
> > I admit I don't understand why PVEC_XWIDGET and PVEC_XWIDGET_VIEW are
> > in the "impossible" cases.  They are first-class Lisp objects, AFAICT.
> 
> If you say so, then they most likely are, indeed.  I personally didn't
> (and still don't) know enough about those to know what to do with them.

Can you tell what are the guidelines for putting a PVEC object into
the "impossible" category in the context of type-of?