bug#62248: 29.0.60; nil obsolete

[Eli Zaretskii <eliz@gnu.org>]

> From: Michael Welsh Duggan <mwd@md5i.com>
> Cc: Michael Welsh Duggan <mwd@md5i.com>,  62248@debbugs.gnu.org,
>   luangruo@yahoo.com
> Date: Sat, 18 Mar 2023 03:15:11 -0400
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > If so, how about running Emacs with a breakpoint in Fput and condition
> > that symbol == Qnil (assuming your Emacs is not configured to
> > represent Lisp objects as structs)?
> 
> I traced it, in my instance, to this line from url-gw.el:
> 
> (make-obsolete-variable url-gw-rlogin-obsolete-warned-once nil "29.1")

OK, I already found (by different means) that one, and fixed it (and
another similar one in url-misc).

> Maybe `make-obsolete-variable' should warn or error with nil as its
> initial argument.

We could install something like that on master, yes.  But then we
should also warn if the symbol is t, I think.