Hacky List Add implementation using context receivers

interface Add<T> {
    operator fun <T1 : T> T1.plus(other: T1): T1
}

object IntAdd : Add<Int> {
    override fun <T1 : Int> T1.plus(other: T1): T1 {
        return (this + other) as T1 // This actually calls the member Int.plus
    }
}

object ListAdd : Add<List<Any?>> {
    override fun <T1 : List<Any?>> T1.plus(other: T1): T1 {
        return buildList(size + other.size) {
            addAll(this@plus)
            addAll(other)
        } as T1
    }
}

context(Add<R>)
fun <T, R, R1: R> Iterable<T>.sumOf(selector: (T) -> R1): R1? {
    var sum: R1? = null
    for (element in this) {
        sum = if (sum == null) {
            selector(element)
        } else {
            sum + selector(element)
        }
    }
    return sum
}
context(Add<T>)
fun <T, T1: T> Iterable<T1>.sum(): T1? {
    return sumOf { it }
}
fun test(){
    with(ListAdd){
        // Intellij shows that this is a List<String>?, success!!
        val bigList = listOf(listOf("hello"), listOf("world"), listOf("this", "actually", "works", "yo")).sum()
        println(bigList)
    }
}