diff -r 035b2afbeb2e -r daaa0b236a3f src/Pure/ML/ml_lex.ML
--- a/src/Pure/ML/ml_lex.ML	Wed Nov 03 11:33:51 2010 +0100
+++ b/src/Pure/ML/ml_lex.ML	Wed Nov 03 13:54:23 2010 +0100
@@ -73,7 +73,14 @@
 fun check_content_of tok =
   (case kind_of tok of
     Error msg => error msg
-  | _ => content_of tok);
+  | _ =>
+      (case tok of
+        Token (_, (Keyword, ":>")) =>
+          warning ("Opaque signature matching (:>) fails to work with ML pretty printing --\n\
+            \prefer non-opaque matching (:) possibly with abstype" ^
+            Position.str_of (pos_of tok))
+      | _ => ();
+      content_of tok));
 
 val flatten = implode o map (Symbol.escape o check_content_of);