src/Pure/axclass.ML

 structure AxClassData = TheoryDataFun
 (struct
   val name = "Pure/axclass";
   type T = axclass Symtab.table * instances;
 
-  val empty = (Symtab.empty, make_instances (Graph.empty, [], []));
+  val empty = (Symtab.empty, make_instances (Graph.empty, [], [])) : T;
   val copy = I;
   val extend = I;
 
   fun merge _ ((axclasses1, instances1), (axclasses2, instances2)) =
     (Symtab.merge (K true) (axclasses1, axclasses2), merge_instances (instances1, instances2));