Definition field_type i m :=
  match Ctypes.field_type i m with
  | Errors.OK t => t
  | _ => Tvoid
  end.Definition field_offset env i m :=
  match Ctypes.field_offset env i m with
  | Errors.OK ofs => ofs
  | _ => 0
  end.Fixpoint field_offset_next_rec env i m ofs sz :=
  match m with
  | nil => 0
  | (i0, t0) :: m0 =>
    match m0 with
    | nil => sz
    | (_, t1) :: _ =>
      if ident_eq i i0
      then align (ofs + @sizeof env t0) (@alignof env t1)
      else field_offset_next_rec env i m0 (align (ofs + @sizeof env t0) (@alignof env t1)) sz
    end
  end.Definition field_offset_next env i m sz := field_offset_next_rec env i m 0 sz.

Lemma in_members_field_type: forall i m, in_members i m -> In (i, field_type i m) m.