IDL43 — May optional be applied to a typedef

The spec is not clear where optional may be applied, I wanted to use it with a typedef, see the test below, but it seems DDS XTypes disallowed it but IDL4 does seem to allow it, it says "This annotation may be used to set optionality on any element that makes sense to be optional.", my example makes sense about it. The spec should be more precise where this annotation (and others) maybe used

module Test {
@optional
typedef long OptLongFoo;
struct Bar

;
};