Improve error signalling for seq-subseq.fix/subsequence-error-with-negative-sequences

The existing behaviour for seq-subseq is to error when indexes are too
large, but to silently ignore numbers which are too negative for lists.
String and vector handling errors in both cases. This has been
regularlised.

Error signalling behaviour has been explicitly added to the docstring of
seq-subseq, and also to cl-subseq which largely defers to
seq-subseq (and is therefore also impacted by this change).

Tests have been added for these exceptional cases, as well as one non
exceptional base case.

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))