bug#65797: `buffer-match-p` should not use `func-arity`

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> I am in favor of this solution as well.

Then how 'bout something like the patch below which changes the
`&optional` into an `&rest` but tries to preserve compatibility with the
old calling convention.


        Stefan


diff --git a/lisp/subr.el b/lisp/subr.el
index e88815fa58c..06c9923b525 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -7295,13 +7292,13 @@ string-lines
             (setq start (length string)))))
       (nreverse lines))))
 
-(defun buffer-match-p (condition buffer-or-name &optional arg)
+(defun buffer-match-p (condition buffer-or-name &rest args)
   "Return non-nil if BUFFER-OR-NAME matches CONDITION.
 CONDITION is either:
 - the symbol t, to always match,
 - the symbol nil, which never matches,
 - a regular expression, to match a buffer name,
-- a predicate function that takes BUFFER-OR-NAME and ARG as
+- a predicate function that takes BUFFER-OR-NAME plus ARGS as
   arguments, and returns non-nil if the buffer matches,
 - a cons-cell, where the car describes how to interpret the cdr.
   The car can be one of the following:
@@ -7326,9 +7323,18 @@ buffer-match-p
                       ((pred stringp)
                        (string-match-p condition (buffer-name buffer)))
                       ((pred functionp)
-                       (if (eq 1 (cdr (func-arity condition)))
-                           (funcall condition buffer-or-name)
-                         (funcall condition buffer-or-name arg)))
+                       (if (cdr args)
+                           ;; More than 1 argument: no need for
+                           ;; Emacs-29 backward compatibility!
+                           (apply condition buffer-or-name args)
+                         (condition-case err
+                             (apply condition buffer-or-name args)
+                           (wrong-number-of-arguments
+                            ;; Backward compatibility with Emacs-29 semantics.
+                            (message "Trying obsolete calling convention for: %S"
+                                     condition)
+                            (apply condition buffer-or-name
+                                   (if args '(nil) nil))))))
                       (`(major-mode . ,mode)
                        (eq
                         (buffer-local-value 'major-mode buffer)
@@ -7350,17 +7356,17 @@ buffer-match-p
                 (throw 'match t)))))))
     (funcall match (list condition))))
 
-(defun match-buffers (condition &optional buffers arg)
+(defun match-buffers (condition &optional buffers &rest args)
   "Return a list of buffers that match CONDITION, or nil if none match.
 See `buffer-match-p' for various supported CONDITIONs.
 By default all buffers are checked, but the optional
 argument BUFFERS can restrict that: its value should be
 an explicit list of buffers to check.
-Optional argument ARG is passed to `buffer-match-p', for
+Optional arguments ARGS are passed to `buffer-match-p', for
 predicate conditions in CONDITION."
   (let (bufs)
     (dolist (buf (or buffers (buffer-list)))
-      (when (buffer-match-p condition (get-buffer buf) arg)
+      (when (apply #'buffer-match-p condition (get-buffer buf) args)
         (push buf bufs)))
     bufs))