src/Pure/type_infer.ML

     val pat_Ts' = map (Type.cert_typ tsig) pat_Ts;
     val is_const = is_some o const_type;
     val raw_ts' =
       map (decode_types tsig is_const def_type def_sort map_const map_type map_sort) raw_ts;
     val (ts, Ts, unifier) = basic_infer_types pp const_type
-      classes arities used freeze is_param raw_ts' pat_Ts';
+      (#2 classes) arities used freeze is_param raw_ts' pat_Ts';
   in (ts, unifier) end;
 
 end;