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

[Alan Mackenzie <acm@muc.de>]

Hello, Eli.

On Sat, Nov 25, 2023 at 17:50:09 +0200, Eli Zaretskii wrote:
> > 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.

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.]

-- 
Alan Mackenzie (Nuremberg, Germany).