bug#70436: 30.0.50; Fail to enter the debugger when using prin1 (instead of cl-prin1)

[Eli Zaretskii]

> From: Bruno Barbier <brubar.cs@gmail.com>
> Date: Wed, 17 Apr 2024 15:29:14 +0200
> 
> When setting debugger-print-function to prin1, Emacs 30.0.50 may fail
> to enter the debugger.
> 
> Emacs displays something like:
> 
>     Entering debugger...
>     make-text-button: Args out of range: 67, 3000
> 
> 
> The correct behavior is to enter the debugger, with something like this:
> 
>     Debugger entered--Lisp error: (wrong-type-argument stringp nil)
>       #f(compiled-function (&rest args) "Start a program in a subprocess.  ...
>       ...
>       make-process(:name "mandatory" :command "ls" :stderr err-buf)
> 
> The problem doesn't occur with Emacs 29.3.
> 
> To manually reproduce, execute the following code block in a new Emacs
> (started with '-Q'):
> 
>   #+begin_src elisp
>     (progn 
>       (setq debugger-print-function 'prin1)
>       
>       (defun my-useless-advice (fun &rest args)
>          (apply fun args))
> 
>       (advice-add 'make-process :around #'my-useless-advice)
> 
>       (make-process :name "mandatory"
>                     :command "ls"
>                     :stderr 'err-buf))
>   #+end_src
>   
> It seems that backtrace.el prints using prin1, but infers where the
> button should be using cl-prin1.
> 
> My personal workaround is to stop setting debugger-print-function to
> prin1 (I customized it a few years ago because I found that cl-prin1 was
> really slow at the time, I guess it shouldn't be the case anymore).

Stefan, is this supposed to be supported?