src/HOL/Tools/record.ML

     val fields = map (apfst full) bfields;
     val names = map fst fields;
     val extN = full bname;
     val types = map snd fields;
     val alphas_fields = fold Term.add_tfree_namesT types [];
-    val alphas_ext = alphas inter alphas_fields;
+    val alphas_ext = inter (op =) (alphas, alphas_fields);
     val len = length fields;
     val variants =
       Name.variant_list (moreN :: rN :: (rN ^ "'") :: wN :: parent_variants)
         (map fst bfields);
     val vars = ListPair.map Free (variants, types);