Kotlin. How to fix: None of following candidates is applicable because of receiver type mismatch

I am studying kotlin and have encountered this problem: Unresolved reference. None of the following candidates is applicable because of receiver type mismatch.

fun main(){
    val arg1 = listOf(1,2,3,4,5,6)
    val arg2 = listOf(2,3,4,5,6,7)

    sumLists(arg1, arg2)
}

fun <T: Number> sumLists(list1: List<T>, list2: List<T>): List<T>?{
    val res = mutableListOf<T>()
    if (list1.size != list2.size)
        return null
    else {
        for (i in 0..list1.size)
            res.add(list1[i] + list2[i])
    }
    return listOf()
}

I don’t really understand why it doesn’t work because I wrote <T: Number>

>Solution :

There is no Number + Number operator defined in Kotlin, so the compiler can’t guarantee that T + T is possible. For example, you can create your own class MyNumber which extends Number, but doesn’t support add operation.

I believe there is no other way than write a separate code for each subtype of Number that you want to support:

for (i in 0 .. list1.lastIndex) {
    val item1 = list1[i]
    val item2 = list2[i]
    @Suppress("UNCHECKED_CAST")
    val result = when {
        item1 is Int && item2 is Int -> item1 + item2
        item1 is Float && item2 is Float -> item1 + item2
        ...
        else -> throw IllegalArgumentException()
    } as T
    res.add(result)
}

We have to do unchecked cast here, because the compiler is not smart enough to understand that the type of the result is T.

Also, I changed for-loop bounds to list1.lastIndex to fix the off-by-one bug.