* lisp/minibuffer.el (completion-at-point): Emit warning for ill-behaved

completion functions.

lisp/ChangeLog

+2014-09-30  Stefan Monnier  <monnier@iro.umontreal.ca>
+
+	* minibuffer.el (completion-at-point): Emit warning for ill-behaved
+	completion functions.
+
 2014-09-29  Stefan Monnier  <monnier@iro.umontreal.ca>
 
 	* ses.el (ses--letref): Quote value before it gets re-evaluated.

lisp/minibuffer.el

@@ -2106,7 +2106,11 @@ The completion method is determined by `completion-at-point-functions'."
          (completion-in-region start end collection
                                (plist-get plist :predicate))))
       ;; Maybe completion already happened and the function returned t.
-      (_ (cdr res)))))
+      (_
+       (when (cdr res)
+         (message "Warning: %S failed to return valid completion data!"
+                  (car res)))
+       (cdr res)))))
 
 (defun completion-help-at-point ()
   "Display the completions on the text around point.