bug#15101: 24.3.50; debugger-eval-expression broken

[Helmut Eller <eller.helmut@gmail.com>]

On Thu, Aug 15 2013, Stefan Monnier wrote:

>> What does "context" mean?
>
> That which is *around*.
>
>> Intuitively I would say that in the second
>> line, ie. the one that looks like "(let ((foo 123)) (debug))", foo is
>> part of the context.
>
> The context in which (let ((foo 123)) (debug)) does not include (yet)
> "foo" (well, maybe it does, but that would be another "foo" than the
> one inside the let).

Consider this example:

(progn
  (defun foo (n)
    (cond ((= n 0) (debug))
      (t (foo (1- n)))))
  (byte-compile 'foo)
  (foo 100))

If we go to the line "foo(10)" I would expect that n is 10. With your
definition n is 11. I would argue that your version is confusing.

>> It seems to me that a better fix would be to adjust linenumber->context
>> mapping by one.
>
> I don't think this can be done, because the semantics become very weird
> and unpredictable.

Can you give an example of what would be weird with this change:

=== modified file 'lisp/emacs-lisp/debug.el'
--- lisp/emacs-lisp/debug.el	2013-08-15 17:21:19 +0000
+++ lisp/emacs-lisp/debug.el	2013-08-16 05:22:02 +0000
@@ -547,7 +547,7 @@
   (interactive
    (list (read--expression "Eval in stack frame: ")))
   (let ((nframe (or nframe
-                    (condition-case nil (1+ (debugger-frame-number 'skip-base))
+                    (condition-case nil (debugger-frame-number 'skip-base)
                       (error 0)))) ;; If on first line.
          (base (if (eq 'debug--implement-debug-on-entry
                       (cadr (backtrace-frame 1 'debug)))


Either way, it would be nice to document why the 1+ was there in the
first place, because it looks like a bug to me.

Helmut