GNU bug report logs - #77464
X is already defined as something else than a generic function

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Message #8 received at 77464 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Jonas Bernoulli <jonas <at> bernoul.li>,
 Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: 77464 <at> debbugs.gnu.org
Subject: Re: bug#77464: X is already defined as something else than a generic
 function
Date: Sun, 13 Apr 2025 11:17:05 +0300

> Date: Wed, 02 Apr 2025 17:59:53 +0200
> From:  Jonas Bernoulli via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs <at> gnu.org>
> 
> Hello,
> 
> Users of my packages occasionally report such errors but I was never
> able to reproduce.  What is strange is that X always was a function that
> was definitely not defined as "something else than a generic function"
> anywhere in my code, or even just in past versions of my code, and users
> were not able to find anything in their configuration or third-party
> packages either.
> 
> Actually I have seen this issue myself a few times, but it was always
> when I badly messed up something else and by the time I had fixed that,
> the "something other" issue was also gone.  In hindsight it is obvious
> that I should sooner have investigated the "something other" issue
> without first fixing the seemingly unrelated issue.
> 
> Now that I have finally done that, I also managed to come of with a
> reproducer.
> 
> 1. Create a file "demo.el" with contents:
> 
> (require 'eieio)
> (defclass demo-class () ())
> (cl-defmethod demo-function ((obj demo-class)) obj)
> 
> (error "something else goes wrong here")
> (provide 'demo)
> 
> 2. Optionally apply this patch to Emacs:
> 
> diff --git a/lisp/emacs-lisp/cl-generic.el b/lisp/emacs-lisp/cl-generic.el
> @@ -193,8 +193,8 @@ cl-generic-ensure-function
>                  (autoloadp (symbol-function name))
>                  (and (functionp name) generic)
>                  noerror)
> -      (error "%s is already defined as something else than a generic function"
> -             origname))
> +      (error "%s [%s] is already defined as something else than a generic function"
> +             origname (symbol-function name)))
>      (if generic
>          (cl-assert (eq name (cl--generic-name generic)))
>        (setf (cl--generic name) (setq generic (cl--generic-make name))))
> @@ -645,7 +645,9 @@ cl-generic-define-method
>            ;; is still valid (e.g. still empty method cache)?
>            (gfun (cl--generic-make-function generic)))
>        (unless (symbol-function sym)
> -        (defalias sym 'dummy))   ;Record definition into load-history.
> +        (defalias sym 'dummy)   ;Record definition into load-history.
> +        (when (eq sym 'demo-function)
> +          (message "First defalias: %s" (symbol-function 'demo-function))))
>        (cl-pushnew `(cl-defmethod . ,(cl--generic-load-hist-format
>                                       (cl--generic-name generic)
>                                       qualifiers specializers))
> @@ -659,7 +661,9 @@ cl-generic-define-method
>            (set-advertised-calling-convention gfun old-adv-cc nil))
>          ;; But do use `defalias', so that it interacts properly with nadvice,
>          ;; e.g. for tracing/debug-on-entry.
> -        (defalias sym gfun)))))
> +        (defalias sym gfun)
> +        (when (eq sym 'demo-function)
> +          (message "Second defalias: %s" (symbol-function 'demo-function)))))))
>  
>  (defvar cl--generic-dispatchers (make-hash-table :test #'equal))
> 
> 3. Paste this into a buffer and adjust the load-path:
> 
> (add-to-list 'load-path "/path/to/demo")
> 
> (message "Load attempt one... [%s]" (symbol-function 'demo-function))
> (with-demoted-errors "Trigger: %S"
>   (require 'demo))
> (message "Load attempt one...done")
> (message "Load attempt two... [%s]" (symbol-function 'demo-function))
> (require 'demo)
> (message "Load attempt two...done")
> 
> 4. Evaluate that buffer, you'll get this output:
> 
> Load attempt one... [nil]
> First defalias: dummy
> Second defalias: #[...]
> Trigger: (error "something else goes wrong here")
> Load attempt one...done
> Load attempt two... [dummy]
> cl-generic-ensure-function: demo-function [dummy] is already defined \
>   as something else than a generic function
> 
> 
> cl-generic-define-method first sets (symbol-function 'demo-function)
> to 'dummy, and then it successfully sets it to something else.  The
> debug statements helped confirm that both defalias calls are actually
> successful, i.e., the problem isn't that something in between these
> calls ends up calling cl-generic-ensure-function.
> 
> Then, after this function has returned, something else goes wrong (in
> this reproducer intentionally).  We can then observe that the value of
> (symbol-function 'demo-function) somehow goes back to being 'dummy.
> (I couldn't find anything else in Emacs that sets a symbol-function to
> 'dummy.)
> 
> The first (require 'demo) failed because the (here intentional) error
> occurred before the provide form was reached.  Once a second attempt to
> require the same library is made, the cl-defmethod fails because the
> encountered 'dummy is "something else than a generic function".
> 
> (Also, isn't it "something OTHER than a generic function"?)

Stefan, any comments or suggestions?

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