Kotlin generic function return type does not conform to declared nullability

Question

fun <T: Any?> function(input: T): T = input
fun function2() {
    val n = function(1)!!
}

Since T is declared as nullable, I expected the output to be nullable as well, but Lint produces Unnecessary non-null assertion (!!) on a non-null receiver of type Int warning. Changing the output type signature to T? makes the warning go away.

Why does the output type not conform to the declaried nullability?

Answer 1

T: Any? does not mean "T is nullable". It constrains the type parameter T to be a subtype of Any?. Nullable types, as well as non-nullable types satisfy this constraint. In general, if B is a subtype of A, then A, B, A?, and B? are all subtypes of A?.

When you do

val n = function(1)

The type parameter T is inferred to be Int, which is a non-nullable type that satisfies the constraint : Any?. The function is declared to return a T, so in this case it returns an Int. There are no problems here, and !! is unnecessary.

Compare that to:

val n = function<Int?>(1)

where you explicitly say that T should be Int?, which is a nullable type (that also satisfy the constraint of : Any). In this case the function returns Int?, and !! can be added without a warning.

Answer 2

T is deduced as Int which is non-nullable

Answer 3

that's because

function(1) --> 1 is not nullable and no need for assertion 

Answer 4

Declaring T as <T: Any?> doesn't mean that T is/has to be nullable. It only means T can be nullable, but doesn't have to be.

If you pass 1 which is Int, then T becomes Int.