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

[Eli Zaretskii]

> Date: Fri, 24 Nov 2023 20:54:45 +0000
> Cc: 67196@debbugs.gnu.org, acm@muc.de
> From: Alan Mackenzie <acm@muc.de>
> 
> > >     Non-nil means enter debugger on an error in a call from 
> > > `eval-expression'.
> > >     Does not apply to errors handled by `condition-case' or those
> > >     matched by `debug-ignored-errors'.
> > >     A nil value for this variable will not prevent an entry to
> > >     the debugger caused by other variables such as `debug-on-error'.
> 
> > First, the last two sentences above should be transposed, as the
> > second one is not related to the 1st one, but the 3rd one is.
> 
> Done.
> 
> > And second, please try to reword so that the text is less complicated
> > and easier to understand.
> 
> I'm perhaps a bit too close to it.  Apart from the first line (for which
> too much information needs squashing in), I can't really see much scope
> for improvement.

Here:

   (defcustom eval-expression-debug-on-error t
    "If non-nil, enter debugger if `eval-expression' signals an error.
  A non-nil value means set `debug-on-error' to that value when calling
  `eval-expression'; this will enter a debugger if `eval-expression'
  signals an error.  A nil value means don't change the value of
  `debug-on-error' when calling `eval-expression'.
  The value can also be a list, with the same meaning as for `debug-on-error'.
  Like  `debug-on-error', this variable does not apply to errors handled
  by `condition-case' or those matched by `debug-ignored-errors'."

> +  DEFVAR_LISP ("debug-from--eval-expression", Vdebug_from__eval_expression,
> +            doc: /* Non-nil means enter debugger if an error is signaled.
> +This only applies in forms called by `eval-expression'.  This variable
> +has the same semantics as `debug-on-error'.  It is an internal variable
> +only.  */);

The doc string should be just

  /* Internal use only, used by `eval-expression'.  */