No, in any particular use of the value both branches return the same type Z.

But when I want to use the value in two different places I don't want Z to be the same in both places.

That's where I'm stuck. When I instantiate the function that represents a value of the sum type I need to choose a return type Z, which is locked in for every use of the value.

I think I understand the idea. I don't see how to make it work in C#.