src/Tools/Code/code_runtime.ML

 
 (* reflection support *)
 
 fun check_datatype thy tyco some_consts =
   let
-    val constrs = (map (fst o fst) o snd o Code.get_type thy) tyco;
+    val constrs = (map fst o snd o fst o Code.get_type thy) tyco;
     val _ = case some_consts
      of SOME consts =>
           let
             val missing_constrs = subtract (op =) consts constrs;
             val _ = if null missing_constrs then []