bug#67196: M-: uses a wrong value of debug-on-error when it is nil.

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 25 Nov 2023 16:40:40 +0000
> Cc: monnier@iro.umontreal.ca, 67196@debbugs.gnu.org, acm@muc.de
> From: Alan Mackenzie <acm@muc.de>
> 
> > > Instead, the new internal variable
> > > debug-from--eval-expression gets set to e-e-debug-on-error.
> 
> > Exactly.  And that value can be non-nil, but also not t.  That is the
> > change I see, and your doc string suggests that it is intentional.
> > Thus, we have a behavior change wrt to the value that the body of
> > eval-expression will see.  And non-nil, non-t values have special
> > meaning there.
> 
> OK, I'm with you, now.  The non-nil, non-t value in the internal variable
> debug-from--e-e won't be seen by any Lisp code.  It can be seen by Lisp
> code in eval-expression-debug-on-error.
> 
> This is an extension of e-e-debug-on-error to match what debug-on-error
> does.  I wouldn't think that d-o-e gets set to a list of conditions very
> often, but when it does, it's likely very useful.  It seemed logical to
> extend the same facility to e-e-debug-on-error.  Do you think this is a
> bad idea?  [Clearly, some entries in NEWS will be needed, as well as
> amendments to the Elisp manual.]

I don't know yet if it is a good idea, I just wanted to point out that
your change is not just a bugfix: it actually changes the behavior of
eval-expression.