bug#68113: Wrong error message triggered in cl--generic-standard-method-combination

[Alan Mackenzie <acm@muc.de>]

Hello, Stefan.

Thanks for the quick reply yesterday.

On Fri, Dec 29, 2023 at 12:24:36 -0500, Stefan Monnier wrote:
> Hi Alan,

> > In my development version of Emacs (based on the master branch) I get a
> > backtrace with the error message being:

> >     Error: wrong-type-argument (symbolp mets-by-qual)

> > This occurs during the execution of cl-generic-combine-methods, whose
> > code starts:

> > (defun cl--generic-standard-method-combination (generic methods)
> >   (let ((mets-by-qual ()))
> >     (dolist (method methods)
> >       (let ((qualifiers (cl-method-qualifiers method)))
> >         (if (eq (car qualifiers) :extra) (setq qualifiers (cddr qualifiers)))
> >         (unless (member qualifiers '(() (:after) (:before) (:around)))
> >           (error "Unsupported qualifiers in function %S: %S"
> >                  (cl--generic-name generic) qualifiers))
> >         (push method (alist-get (car qualifiers) mets-by-qual))))

> > It is the last line that is signalling the error.  The pertinent line
> > from the backtrace which is the expansion of that last line reads:

> >     (let* ((k (car qualifiers)) (p (assq k mets-by-qual)) (v (cons method (cdr p)))) (progn (if p (setcdr p v) (progn (signal 'wrong-type-argument (list 'symbolp 'mets-by-qual)))) v))

> > ..  The error is, in actual fact, the failure to find an entry for (car
> > qualifiers) in the alist mets-by-qual.  The error message given is
> > rubbish and more than a little misleading.  mets-by-qual is clearly a
> > symbol.

> [ Side note: not finding an entry for (car qualifiers) in the above code
>   is perfectly normal (it's the most common case, even).  The code only
>   finds such an entry when there are several applicable methods (and
>   they have the same set of qualifiers).  ]

> Hmm... the error occurs during macroexpansion, because the
> macroexpansion of the `push` above should be (and is, normally):

>     ELISP> (macroexpand '(push method (alist-get (car qualifiers) mets-by-qual)))
>     (let*
>         ((k (car qualifiers)) (p (assq k mets-by-qual))
>          (v (cons method (cdr p))))
>       (progn
>         (if p (setcdr p v)
>           (setq mets-by-qual (cons (setq p (cons k v)) mets-by-qual)))
>         v))
    
This was a good hint, thanks.  The most likely source of the (signal
.....) form would seem to be the clause handling `setq' in
macroexp--expand-all.  I suspected it might be caused, somehow, by
symbolp not recognising symbols with position as symbols.  But I
tightened up all the entry points, and disabled SWPs, and I still can't
get the code in macroexp--expand-all to printf a message for
mets-by-qual.

>     ELISP>

> I don't know why you're not getting that expansion, and I don't know
> either why you're getting that

>     (signal 'wrong-type-argument (list 'symbolp 'mets-by-qual))

I don't know, either.  :-(  As I say, I've tried instrumenting the `setq'
handling code in macroexp--expand-all, but haven't managed to get
anything pertinent output.

> AFAICT this weird code is likely generated by `gv-delay-error` but
> according to `grep` it's only used in `map-elt` so I can't see why it's
> showing up here.

> I'd start debugging this with something like `M-x trace-function RET
> gv-get RET` and `M-x debug-on-entry RET gv-delay-error RET`.
> [ Tho, presumably you're seeing this during the bootstrap, so you'll
>   probably want to add `message/debug` calls in the code instead.  ]

I am indeed seeing this in bootstrap, so it's message and a bit of prin1.

>         Stefan

-- 
Alan Mackenzie (Nuremberg, Germany).