bug#60464: 29.0.60; Regression - pcomplete-arg fails with argument 'last

[Gregory Heytings]

> > I'm not the one who wrote that code.  According to my analysis of the 
> > code, the only place where pcomplete-arg is called with a 'last' 
> > argument is pcomplete-parse-arguments, where you will see the 
> > following:
> >
> > (defun pcomplete-parse-arguments (&optional expand-p)
> >   ...
> >   (let ((results (funcall pcomplete-parse-arguments-function)))
> >     (when results
> >       (setq ...
> >             pcomplete-stub (pcomplete-arg 'last))
> >       (let ...
> >         (if (and (listp pcomplete-stub) ;??
> >                  (not pcomplete-expand-only-p))
> >             ;; If `pcomplete-stub' is a list, it means it's a list of
> >             ;; completions computed during parsing, e.g. Eshell uses
> >             ;; that to turn globs into lists of completions.
>
> That's also my understanding.  So I think The Right Fix (or at least The 
> Better Fix) is to pay no special attention to `last` in `pcomplete-arg` 
> and instead in the above code of `pcomplete-parse-arguments` to look for 
> the `pcomplete-arg-value` property.

That would be even better, indeed.  But it would be a larger change.

> Maybe for `emacs-29` we can use your patch (with a comment about why 
> `last` is handled specially pointing to its handling in 
> `pcomplete-parse-arguments`) and then in `master` we remove this special 
> handling of `last`?

I'm fine with that.

Unless someone objects, I'll push that patch in a day or two, and I'll do 
that change on master afterwards.