diff -r b28defd0b5a5 -r db75b4005d9a src/HOL/Tools/Quickcheck/quickcheck_common.ML
--- a/src/HOL/Tools/Quickcheck/quickcheck_common.ML	Mon Jul 16 15:57:21 2012 +0200
+++ b/src/HOL/Tools/Quickcheck/quickcheck_common.ML	Mon Jul 16 21:20:56 2012 +0200
@@ -449,7 +449,7 @@
       (Datatype_Aux.interpret_construction descr vs constr)
     val insts = insts_of sort  { atyp = single, dtyp = (K o K o K) [] }
       @ insts_of aux_sort { atyp = K [], dtyp = K o K }
-    val has_inst = exists (fn tyco => can (Sorts.mg_domain algebra tyco) sort) tycos;
+    val has_inst = exists (fn tyco => Sorts.has_instance algebra tyco sort) tycos;
   in if has_inst then thy
     else case perhaps_constrain thy insts vs
      of SOME constrain => instantiate config descr