Merge remote-tracking branch 'origin/fix/subsequence-error-with-negative-sequences'

2 files changed, 9 insertions, 2 deletions

diff --git a/lisp/emacs-lisp/cl-extra.el b/lisp/emacs-lisp/cl-extra.el
index 101864d3721..9742014db0c 100644
--- a/lisp/emacs-lisp/cl-extra.el
+++ b/lisp/emacs-lisp/cl-extra.el
@@ -518,7 +518,9 @@ This sets the values of: `cl-most-positive-float', `cl-most-negative-float',
 (defun cl-subseq (seq start &optional end)
   "Return the subsequence of SEQ from START to END.
 If END is omitted, it defaults to the length of the sequence.
-If START or END is negative, it counts from the end."
+If START or END is negative, it counts from the end.
+Signal an error if START or END are outside of the sequence (i.e
+too large if positive or too small if negative)"
   (declare (gv-setter
             (lambda (new)
               (macroexp-let2 nil new new
diff --git a/lisp/emacs-lisp/seq.el b/lisp/emacs-lisp/seq.el
index 9eed36eb68c..038b20e3b5e 100644
--- a/lisp/emacs-lisp/seq.el
+++ b/lisp/emacs-lisp/seq.el
@@ -221,12 +221,17 @@ TESTFN is used to compare elements, or `equal' if TESTFN is nil."
 (defun seq-subseq (seq start &optional end)
   "Return the subsequence of SEQ from START to END.
 If END is omitted, it defaults to the length of the sequence.
-If START or END is negative, it counts from the end."
+If START or END is negative, it counts from the end.
+
+Signal an error if START or END are outside of the sequence (i.e
+too large if positive or too small if negative)"
   (cond ((or (stringp seq) (vectorp seq)) (substring seq start end))
         ((listp seq)
          (let (len (errtext (format "Bad bounding indices: %s, %s" start end)))
            (and end (< end 0) (setq end (+ end (setq len (seq-length seq)))))
            (if (< start 0) (setq start (+ start (or len (setq len (seq-length seq))))))
+           (unless (>= start 0)
+             (error "%s" errtext))
            (when (> start 0)
              (setq seq (nthcdr (1- start) seq))
              (or seq (error "%s" errtext))