(backquote-process): Don't crash if ultimate

expression is just a variable.

1 files changed, 20 insertions, 7 deletions

diff --git a/lisp/emacs-lisp/backquote.el b/lisp/emacs-lisp/backquote.el
index 777cbbf8f9a..d67e2bef6f4 100644
--- a/lisp/emacs-lisp/backquote.el
+++ b/lisp/emacs-lisp/backquote.el
@@ -141,8 +141,18 @@ Vectors work just like lists.  Nested backquotes are permitted."
    ((eq (car s) backquote-backquote-symbol)
     (backquote-process (cdr (backquote-process (nth 1 s)))))
    (t
-    (let ((rest s) (item nil) (firstlist nil) (list nil) (lists nil))
+    (let ((rest s)
+          item firstlist list lists expression)
+      ;; Scan this list-level, setting LISTS to a list of forms,
+      ;; each of which produces a list of elements
+      ;; that should go in this level.
+      ;; The order of LISTS is backwards. 
+      ;; If there are non-splicing elements (constant or variable)
+      ;; at the beginning, put them in FIRSTLIST,
+      ;; as a list of tagged values (TAG . FORM).
+      ;; If there are any at the end, they go in LIST, likewise.
       (while (consp rest)
+        ;; Turn . (, foo) into (,@ foo).
         (if (eq (car rest) backquote-unquote-symbol)
             (setq rest (list (list backquote-splice-symbol (nth 1 rest)))))
         (setq item (backquote-process (car rest)))
@@ -158,20 +168,23 @@ Vectors work just like lists.  Nested backquotes are permitted."
          (t
           (setq list (cons item list))))
         (setq rest (cdr rest)))
+      ;; Handle nonsplicing final elements, and the tail of the list
+      ;; (which remains in REST).
       (if (or rest list)
           (setq lists (cons (backquote-listify list (backquote-process rest))
                             lists)))
-      (setq lists
+      ;; Turn LISTS into a form that produces the combined list. 
+      (setq expression
             (if (or (cdr lists)
-                    (and (consp (car lists))
-                         (eq (car (car lists)) backquote-splice-symbol)))
+                    (eq (car-safe (car lists)) backquote-splice-symbol))
                 (cons 'append (nreverse lists))
               (car lists)))
+      ;; Tack on any initial elements.
       (if firstlist
-          (setq lists (backquote-listify firstlist (cons 1 lists))))
-      (if (eq (car lists) 'quote)
+          (setq expression (backquote-listify firstlist (cons 1 expression))))
+      (if (eq (car-safe expression) 'quote)
           (cons 0 (list 'quote s))
-        (cons 1 lists))))))
+        (cons 1 expression))))))
 
 ;; backquote-listify takes (tag . structure) pairs from backquote-process
 ;; and decides between append, list, backquote-list*, and cons depending