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

[Eli Zaretskii]

> Date: Sat, 25 Nov 2023 14:14:59 +0000
> Cc: monnier@iro.umontreal.ca, 67196@debbugs.gnu.org, acm@muc.de
> From: Alan Mackenzie <acm@muc.de>
> 
> > No, I meant with the current code debug-on-error is set to t whenever
> > eval-expression-debug-on-error is non-nil, ....
> 
> Yes.
> 
> > .... and your change sets it to the same value as
> > eval-expression-debug-on-error instead.
> 
> That is not what is meant to happen.

We are miscommunicating.

> 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.