[Emacs-diffs] fix/subsequence-error-with-negative-sequences feadec3: Improve error signalling for seq-subseq.

[Phillip Lord]

branch: fix/subsequence-error-with-negative-sequences
commit feadec307da148af70cf87013c99771ca4db91e4
Author: Phillip Lord <address@hidden>
Commit: Phillip Lord <address@hidden>

    Improve error signalling for seq-subseq.
    
    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.
---
 lisp/emacs-lisp/cl-extra.el |    4 +++-
 lisp/emacs-lisp/seq.el      |    7 ++++++-
 test/automated/seq-tests.el |    7 ++++++-
 3 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/lisp/emacs-lisp/cl-extra.el b/lisp/emacs-lisp/cl-extra.el
index 101864d..9742014 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 9eed36e..038b20e 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))
diff --git a/test/automated/seq-tests.el b/test/automated/seq-tests.el
index 3643ce5..74c0700 100644
--- a/test/automated/seq-tests.el
+++ b/test/automated/seq-tests.el
@@ -187,7 +187,12 @@ Evaluate BODY for each created sequence.
   (should-not   (seq-subseq '(1 2 3) 3))
   (should       (seq-subseq '(1 2 3) -3))
   (should-error (seq-subseq '(1 2 3) 1 4))
-  (should       (seq-subseq '(1 2 3) 1 3)))
+  (should       (seq-subseq '(1 2 3) 1 3))
+  (should-error (seq-subseq '() -1))
+  (should-error (seq-subseq [] -1))
+  (should-error (seq-subseq "" -1))
+  (should-not (seq-subseq '() 0))
+  (should-error(seq-subseq '() 0 -1)))
 
 (ert-deftest test-seq-concatenate ()
   (with-test-sequences (seq '(2 4 6))