Context {cs: compspecs}.
Open Scope Z.
Definition co_default (s: struct_or_union): composite.
apply (Build_composite s nil noattr 0 1 0).
+ intro. inv H.
+ exists 0%nat; auto.
+ exists 0; auto.
Defined.
Definition get_co id :=
match cenv_cs ! id with
| Some co => co
| _ => co_default Struct
end.

Lemma sizeof_Tunion: forall id a,
sizeof (Tunion id a) = co_sizeof (get_co id).