if I do a

when

over a type system, like when { foo is A -> foo is B -> } why can the compiler not figure out compile-time if the when is exhaustive without an else branch?

It can if you use a sealed class.

Because you cannot know about all subclasses unless it's a sealed class. Someone can take your module as a dependency, and inherit given class.

You should do a

when (foo)

instead:

sealed class Foo

object A : Foo()
object B : Foo()

fun main() {
    val foo: Foo = A

//  This won't work, as there are other
//  potential cases that doesn't involve 
// `foo`.
//  val bar = when {
//      foo is A -> TODO()
//      foo is B -> TODO()
//  }

    val bar = when (foo) {
        is A -> TODO()
        is B -> TODO()
    }
}