src/Pure/theory.ML

--- a/src/Pure/theory.ML	Fri Jul 04 17:19:03 2014 +0200
+++ b/src/Pure/theory.ML	Fri Jul 04 17:41:35 2014 +0200
@@ -262,7 +262,8 @@
     else if Type.raw_instance (declT, T') then
       error (message true "imposes additional sort constraints on the constant declaration")
     else if overloaded then ()
-    else warning (message false "is strictly less general than the declared type")
+    else
+      error (message false "is strictly less general than the declared type (overloading required)")
   end;