It would be great to have union types just for external declarations in K/JS, so it's a lot easier to create typings for a TypeScript library. I think it would be a pretty easy task to implement it just in that specific context so this is why I post a message specifically for this

It won't be easy, because it means full support of union types in type system, which is main blocker from design and implementation sides

Ah, but doesn't they exist in a way when using multiple return types in a

when

?

No.

when

expressions are nothing more than

select

function from inference point of view

when {
  ... -> x
  ... -> y
  ... -> z
}
// same as
fun <K> select(vararg value: K): K

select(x, y, z)

Ah okay I see, thanks for the information :)