src/Pure/type.ML

   let
     val log_types =
       Symtab.fold (fn (c, (LogicalType n, _)) => cons (c, n) | _ => I) (snd types) []
       |> Library.sort (Library.int_ord o pairself snd) |> map fst;
     val witness =
-      (case Sorts.witness_sorts classes log_types [] [Sorts.classes classes] of
+      (case Sorts.witness_sorts classes log_types [] [Sorts.minimal_classes classes] of
         [w] => SOME w | _ => NONE);
   in make_tsig ((space, classes), default, types, log_types, witness) end;
 
 fun map_tsig f (TSig {classes, default, types, log_types = _, witness = _}) =
   build_tsig (f (classes, default, types));