type_ternary env ctx cond tru fls : Prog.Expression.t =
  let cond_typed = type_expression env ctx cond in
  let open Expression in 
  assert_bool (tags cond) cond_typed.typ |> ignore;
  let fls_typed = type_expression env ctx fls in
  let tru_typed = type_expression env ctx tru in
  assert_same_type env (tags tru) (tags fls) tru_typed.typ fls_typed.typ |> ignore;
  match tru_typed.typ with
  | Type.Integer ->
     (* TODO this is allowed if cond is compile-time known *)
     failwith "Conditional expressions may not have arbitrary width integer type"
  | t ->
     { expr = Ternary { cond = cond_typed; tru = tru_typed; fls = fls_typed; tags = Info.dummy };
       typ = t;
       dir = Directionless;
       tags = Info.dummy}