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

[Alan Mackenzie]

Hello, Stefan.

On Sun, Nov 19, 2023 at 12:19:49 -0500, Stefan Monnier wrote:
> > Thinking about it more clearly, we're using one variable, debug-on-error,
> > for two conflicting purposes:
> > (i) The calling mechanism of execute-extended-command, including the
> >   invocation of the debugger, should an error be signalled.
> > (ii) Use in the form to be evaluated, where the variable's value outside
> >   of execute-extended-command should endure.

> Yes, there's a conflicting need here: we want to control the way `eval`
> works "from the outside", i.e. we want the `debug-on-error` to apply to
> the evaluator but not to the code being evaluated.

> But `eval` doesn't distinguish between the context of the evaluator and
> the context of the code being evaluated because all ELisp code uses the
> same evaluator.  I can't think of any way to split the variable into two
> to solve the problem.  Instead it gives me the impression we're trying
> some impossible feat like watching oneself sleep.

I can think of a way to do this (indeed, I've got a trial implementation
here which works).

Firstly, though, there is a bug in the doc string of
eval-expression-debug-on-error: rather than stating what the meaning of
the variable is, what it's for, it states the low level details of how
it achieves the desired effect.  This is needlessly restrictive.  I
propose changing that doc string from:

    If non-nil set `debug-on-error' to t in `eval-expression'.
    If nil, don't change the value of `debug-on-error'.

to something like:

    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'.
    Like `debug-on-error', this variable's value can also be a list,
    with the same meaning as for `debug-on-error'.    

..  With this change, the mechanism for eval-expression-debug-on-error
can be changed such that it doesn't become confused with debug-on-error.

>         Stefan

-- 
Alan Mackenzie (Nuremberg, Germany).