Best way to Strong-Type a primitive in Kotlin

Question

Following similar patterns in other languages, I would be interested in producing the most useful way to strongly-type a primitive type in Kotlin.

The rationale, of course, is to have two types which are basically primitive (e.g. strings), but which cannot be assignable to each other by mistake.

My latest attempt is given here, and I'm interested to know how can it be minimized further (can defining the derived constructor be omitted?)

abstract class StronglyTyped<T>{
    private var value: T
    constructor(_value: T) {
        value = _value
    }
    operator fun invoke(): T {
        return value
    }
}

class UserId: StronglyTyped<String> {
    constructor(_value: String): super(_value) {}
}

class UserName: StronglyTyped<String> {
    constructor(_value: String): super(_value) {}
}

fun main() {
    val a = UserId("this is a userId")
    val b = UserName("this is a userName")
    var c: UserName
    //c = a // <== won't compile
    c = b
    println(c())
}