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

[Helmut Eller]

On Thu, Aug 15 2013, Stefan Monnier wrote:

>> Start Emacs with: emacs -Q --eval '(let ((foo 123)) (debug))'
>> Then press e. Enter foo to evaluate the local variable foo. But it
>> doesn't work; it only generates the error:
>> debugger-eval-expression: Symbol's value as variable is void: foo
>> This used to work fine in previous versions.
>
> Indeed, this is a change that will trip up users.  Here's what's
> happening: `e' will now run the code in the context in which the "code
> on the current line" was run.
>
> This refinement can be useful for dynamically bound variables, but was
> mostly added for lexically bound variables, where it's indispensable.
>
> So the above recipe works again if you use C-p before `e' so that point
> is now on the top-most line, which stands for "in the context that
> called `debug'".
>
> I think a good fix is to change debug.el so that point starts on the
> first line of the *Debugger* buffer rather than on the second.

What does "context" mean?  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.

Compare this with:
(let ((foo 1))
  (let ((bar 2))
    (let ((baz 3))
      (debug))))

It seems to me that a better fix would be to adjust linenumber->context
mapping by one.

Helmut