interface A {
    fun getInt(): Int
}

interface B {
    fun getString(): String
}

sealed class Union<TA, TB> {
    class Left<T1, T2>(val value: T1) : Union<T1, T2>()
    class Right<T1, T2>(val value: T2) : Union<T1, T2>()
}

fun A.union(): Union<A, B> {
    return Union.Left(this)
}

fun B.union(): Union<A, B> {
    return Union.Right(this)
}

fun kotlinTypeUnion(param: Union<A, B>) {
    println(
        when (param) {
            is Union.Left -> param.value.getInt() // .value is an A
            is Union.Right -> param.value.getString() // .value is a B
        }
    )
}

fun main() {

    kotlinTypeUnion(
        object : A {
            override fun getInt(): Int {
                return 123
            }
        }.union()
    )

    kotlinTypeUnion(
        object : B {
            override fun getString(): String {
                return "str"
            }
        }.union()
    )
}