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

[Eli Zaretskii]

> Date: Wed, 15 Nov 2023 13:48:40 +0000
> From: Alan Mackenzie <acm@muc.de>
> 
> Hello, Emacs.
> 
> On the master branch (and probably any recent Emacs):
> 
> (i) emacs -Q
> (ii) C-h v debug-on-error RET
> (iii) M-: debug-on-error RET
> 
> >From (ii) it will be seen that debug-on-error's value is nil.
> (iii) wrongly reports that its value is t.  This is a bug.
> 
> #########################################################################
> 
> What is happening here is that eval-expression binds debug-on-error to
> an uninterned symbol called "t", so that d-o-e will be set to non-nil
> for the evaluation of the coming form.
> 
> This has the unwanted side effect of ignoring the actual value of d-o-e
> in forms which use it.  For example, if the variable is set to a list of
> error symbols, this value gets ignored on evaluating a form with M-:.

Aren't we doing this on purpose, Stefan?