Fix imenu--sort-by-position for non-pairs parameters (bug#26457)

* lisp/imenu.el (imenu--sort-by-position): Fix to accept lists beyond
  pairs.
* test/lisp/imenu-tests.el: Add 2 tests for `imenu--sort-by-position`.

1 files changed, 6 insertions, 1 deletions

diff --git a/lisp/imenu.el b/lisp/imenu.el
index 0f47a92734e..c1fd4005ab6 100644
--- a/lisp/imenu.el
+++ b/lisp/imenu.el
@@ -499,7 +499,12 @@ An item looks like (NAME . POSITION)."
   (string-lessp (car item1) (car item2)))
 
 (defun imenu--sort-by-position (item1 item2)
-  (< (cdr item1) (cdr item2)))
+  "Comparison function to sort items depending on their position.
+Return non-nil if and only if ITEM1's position is lower than ITEM2's
+position."
+  (if (listp (cdr item1))
+      (< (cadr item1) (cadr item2))
+    (< (cdr item1) (cdr item2))))
 
 (defun imenu--relative-position (&optional reverse)
   "Support function to calculate relative position in buffer.