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 12:40:03 +0000
> Cc: monnier@iro.umontreal.ca, 67196@debbugs.gnu.org, acm@muc.de
> From: Alan Mackenzie <acm@muc.de>
> 
> > What I had in mind was a function exposed to Lisp that would set an
> > internal variable not exposed to Lisp.
> 
> The would still require an unwind-protect somewhere.

We can unwind-protect in C as well.

> > > Sorry, I wasn't clear enough.  During those 10 hours, I was under the
> > > impression that debug-on-error was t, because M-: debug-on-error said so.
> > > It actually was nil.  That's why I submitted this bug report.
> 
> > So maybe instead of changing how this stuff works we should improve
> > how debug-on-error's value is reported by M-: and other eval commands?
> 
> Yes, but that might be complicated, and won't help the user trying to
> debug something which depends on debug-on-error, who is using M-: to try
> to test it.

I don't see how that could happen.

> > Note that (AFAIU) your change doesn't just solve the problem you
> > bumped into, it also changes the value of debug-on-error inside
> > eval-expression etc., when eval-expression-debug-on-error's value is
> > non-nil, but not t.  I wonder what is the reason for that?
> 
> I don't see that in my current version of the patch (below).  To test
> this, I used the following:
> 
> (defun foo ()
>   (interactive)
>   (message "debug-on-error is %s" debug-on-error)
>   (message "eval-expression-debug-on-error is %s" eval-expression-debug-on-error)
>   (car 'foo))
> 
> , and called it with various settings of debug-on-error and
> eval-expression-debug-on-error.  In particular, with
> 
>   (setq eval-expression-debug-on-error '(wrong-type-argument))
> 
> , I still see debug-on-error reported as nil.

No, I meant with the current code debug-on-error is set to t whenever
eval-expression-debug-on-error is non-nil, and your change sets it to
the same value as eval-expression-debug-on-error instead.