Subject: bug#68938: Emacs "master". Incorrect code generated by pcase.

From: Alan Mackenzie <[email-masked]>
Date: Mon, 5 Feb 2024 17:18:40 +0000

Hello Stefan and Emacs.

In a development version of Emacs, last synched with master in December,
I have added the following pcase clause to macroexp--expand-all in
lisp/emacs-lisp/macroexp.el:

            (`(,(and 'defalias d
                     (guard (and (null defining-symbol)
                                 (symbol-with-pos-p d))))
               ',sym . ,_)
             ;; Here, don't change the form; just set `defining-symbol'
             ;; for a (defalias 'foo ...) in the source code.
             ;; (when (symbol-with-pos-p d)
             (setq defining-symbol sym)
             form)

..  pcase expands that clause to this cond clause:

((eq x0 'defalias)
 (cond
  ((let* ((d x0))
     (and (null defining-symbol) (symbol-with-pos-p d)))
   (let* ((x14 (cdr-safe form)))
     (cond
      ((consp x14)
       (let* ((x15 (car-safe x14)))
         (cond
          ((consp x15)
           (let* ((x16 (car-safe x15)))
             (cond
              ((eq x16 'quote)
               (let* ((x17 (cdr-safe x15)))
                 (cond
                  ((consp x17)
                   (let* ((x18 (car-safe x17)) (x19 (cdr-safe x17)))
                     (cond
                      ((null x19)
                       (let ((d x0) (sym x18))
                         (ignore d) (setq defining-symbol sym) form))
                      ((consp x0)
                       (let* ((x21 (car-safe x0)))
                         (if (eq x21 'lambda) (funcall pcase-3 x0 x14)
                           (funcall pcase-2 x0))))
                      (t (funcall pcase-2 x0)))))
                  ((consp x0)
                   (let* ((x23 (car-safe x0)))
                     (if (eq x23 'lambda) (funcall pcase-3 x0 x14)
                       (funcall pcase-2 x0))))
                  (t (funcall pcase-2 x0)))))
              ((consp x0)
               (let* ((x25 (car-safe x0)))
                 (if (eq x25 'lambda) (funcall pcase-3 x0 x14)
                   (funcall pcase-2 x0))))
              (t (funcall pcase-2 x0)))))
          ((consp x0)
           (let* ((x27 (car-safe x0)))
             (if (eq x27 'lambda) (funcall pcase-3 x0 x14)
               (funcall pcase-2 x0))))
          (t (funcall pcase-2 x0)))))
      ((consp x0)
       (let* ((x29 (car-safe x0)))
         (if (eq x29 'lambda) (funcall pcase-3 x0 x14) (funcall pcase-2 x0))))
      (t (funcall pcase-2 x0)))))
  ((consp x0)
   (let* ((x31 (car-safe x0)))
     (if (eq x31 'lambda)
         (let* ((x33 (cdr-safe form))) (funcall pcase-3 x0 x33))
       (funcall pcase-2 x0))))
  (t (funcall pcase-2 x0))))

..  This contains errors:

(i) Although it has been established that x0 is 'defalias, there are many
  tests (consp x0).
(ii) There are calls of the form (funcall pcase-2 x0), i.e. (funcall
  pcase-2 'defalias).  This causes a wrong-number-of-arguments error.
(iii) There is no sign of the final `form' being returned, though this
  may be being done elsewhere.

-- 
Alan Mackenzie (Nuremberg, Germany).