src/Pure/Isar/proof_context.ML

 fun check_class ctxt (xname, pos) =
   let
     val tsig = tsig_of ctxt;
     val class_space = Type.class_space tsig;
 
-    val name = Type.cert_class tsig (Type.intern_class tsig xname)
+    val name = Type.cert_class tsig (Name_Space.intern class_space xname)
       handle TYPE (msg, _, _) =>
         error (msg ^ Position.here pos ^
           Markup.markup_report (Completion.reported_text
             (Name_Space.completion (Context.Proof ctxt) class_space (xname, pos))));
     val reports =