>sealed hierarchies for sums

I wouldn't call that a good example of sum types. You can use sealed classes or other mechanism in C# to emulate sum types. I was thinking about something built in the language.

Example:

type IntOrBool = | I of int | B of bool

It's built into the language and the VM (the VM enforces the "sealedness", and pattern matching uses it for exhaustiveness checks). It's just that all types in Java are nominal.