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

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> +  (let* ((debug-from--eval-expression eval-expression-debug-on-error)
> +         (result (values--store-value
> +                  (eval (let ((lexical-binding t)) (macroexpand-all exp)) t)))
> +         (print-length (unless no-truncate eval-expression-print-length))
> +         (print-level  (unless no-truncate eval-expression-print-level))
> +         (eval-expression-print-maximum-character char-print-limit)
> +         (deactivate-mark)
> +         (out (if insert-value (current-buffer) t)))
> +    (prog1
> +        (prin1 result out)
> +      (let ((str (and char-print-limit
> +                      (eval-expression-print-format result))))
> +        (when str (princ str out))))))

So you kicked the can a bit further down the road.
The next bug report will be that `M-: debug-from--eval-expression` does
not return the expected value.

FWIW, the other way I came up to circumvent the problem is to test the
shape of the expression to evaluate and only use
`eval-expression-debug-on-error` when the expression is not a mere
symbol (for which the backtrace would presumably not be interesting anyway).
But then we get the weird situation where `M-x debug-on-error` can
return nil but `M-x (list debug-on-error ...)` returns a list that
starts with t.

I think I'd rather keep the current code, whose semantics is
actually simpler.


        Stefan