src/HOL/Tools/datatype_abs_proofs.ML

         val n = Sign.arity_number thy tyco;
       in
         thy
         |> Class.prove_instance_arity (Class.intro_classes_tac [])
             [(tyco, replicate n HOLogic.typeS, [HOLogic.class_size])] []
+        |> snd
       end
 
     val (size_def_thms, thy') =
       thy1
       |> Theory.add_consts_i (map (fn (s, T) =>