[gavinking]

Sure. So I'll give you one very over-simplistic example. Let's assume that T is an unknown type (a type parameter).

You can form types like Object&T, and if T is an optional type of form S|Null, then that is reduced automatically to the type S by the typechecker. But if T is not an optional type, then it just remains T.

Very useful to be able to do this kind of trick in generic code.

And note that nothing here was using any special reasoning about Null: this is all just the totally generic reasoning for union types in general.