Subject: 29.1; Anomaly in cl-destructuring-bind &rest handling

From: Lassi Kortela <[email-masked]>
Date: Sat, 2 Dec 2023 02:38:41 +0200

;; These three return the first value to which `a' was bound, which is
;; probably OK.

(cl-destructuring-bind (a &rest a) (list 1 2 3 4)
   a)
;; => 1

(cl-destructuring-bind (b a &rest a) (list 1 2 3 4)
   a)
;; => 2

(cl-destructuring-bind (a b &rest b) (list 1 2 3 4)
   a)
;; => 1

;; These two produce the anomaly. When the &rest parameter has the
;; same variable same as a positional parameter, and there are at
;; least two positional parameters, then &rest tries to decode the
;; list tail from whatever value was first assigned to the variable by
;; a positional parameter.

(cl-destructuring-bind (a b &rest a) (list 1 2 3 4)
   a)
;; error: (wrong-type-argument listp 1)

(cl-destructuring-bind (a a &rest a) (list 1 2 3 4)
   a)
;; error: (wrong-type-argument listp 1)