* lisp/simple.el (region-bounds): New function.

(region-noncontiguous-p): Use it.
http://lists.gnu.org/archive/html/emacs-devel/2017-01/msg00044.html

etc/NEWS

@@ -787,6 +787,11 @@ of an arbitrary function.  This generalizes 'subr-arity' for functions
 that are not built-in primitives.  We recommend using this new
 function instead of 'subr-arity'.
 
+** New function 'region-bounds' can be used in the interactive spec
+to provide region boundaries (for rectangular regions more than one)
+to an interactively callable function as a single argument instead of
+two separate arguments region-beginning and region-end.
+
 +++
 ** 'parse-partial-sexp' state has a new element.  Element 10 is
 non-nil when the last character scanned might be the first character

lisp/simple.el

@@ -5410,11 +5410,15 @@ also checks the value of `use-empty-active-region'."
        ;; region is active when there's no mark.
        (progn (cl-assert (mark)) t)))
 
+(defun region-bounds ()
+  "Return the boundaries of the region as a list of (START . END) positions."
+  (funcall region-extract-function 'bounds))
+
 (defun region-noncontiguous-p ()
   "Return non-nil if the region contains several pieces.
 An example is a rectangular region handled as a list of
 separate contiguous regions for each line."
-  (> (length (funcall region-extract-function 'bounds)) 1))
+  (> (length (region-bounds)) 1))
 
 (defvar redisplay-unhighlight-region-function
   (lambda (rol) (when (overlayp rol) (delete-overlay rol))))