> You can't have closed enums without value constraints

Sum types and closed enums don't need to constrain existing sets of values, they define the set of values. Again, I think you might be confusing the type system with runtime representation.

> It's a neat parlour trick, don't get me wrong,

It's a step towards sum types which are the mathematical dual of product types. Not a parlour trick at all, every modern language should have algebraic data types.