Fix circular list handling in seq-mapn

* lisp/emacs-lisp/seq.el (seq-mapn): Do not copy list arguments.
* test/lisp/emacs-lisp/seq-tests.el (test-seq-mapn-circular-lists):
  Add a regression test.

2 files changed, 9 insertions, 1 deletions

diff --git a/lisp/emacs-lisp/seq.el b/lisp/emacs-lisp/seq.el
index 5ddc5a53a32..9890e60614e 100644
--- a/lisp/emacs-lisp/seq.el
+++ b/lisp/emacs-lisp/seq.el
@@ -178,7 +178,10 @@ Return a list of the results.
 
 \(fn FUNCTION SEQUENCES...)"
   (let ((result nil)
-        (sequences (seq-map (lambda (s) (seq-into s 'list))
+        (sequences (seq-map (lambda (s)
+                              (if (listp s)
+                                  s
+                                (seq-into s 'list)))
                             (cons sequence sequences))))
     (while (not (memq nil sequences))
       (push (apply function (seq-map #'car sequences)) result)
diff --git a/test/lisp/emacs-lisp/seq-tests.el b/test/lisp/emacs-lisp/seq-tests.el
index 2e533acce17..fc65c9834d6 100644
--- a/test/lisp/emacs-lisp/seq-tests.el
+++ b/test/lisp/emacs-lisp/seq-tests.el
@@ -386,5 +386,10 @@ Evaluate BODY for each created sequence.
   (should-error (seq-random-elt []))
   (should-error (seq-random-elt "")))
 
+(ert-deftest test-seq-mapn-circular-lists ()
+  (let ((l1 '#1=(1 . #1#)))
+    (should (equal (seq-mapn #'+ '(3 4 5 7) l1)
+                   '(4 5 6 8)))))
+
 (provide 'seq-tests)
 ;;; seq-tests.el ends here