Kotlin generic function return type does not conform to declared nullability-CodePudding

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?

CodePudding user response：

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.

CodePudding user response：

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.

CodePudding user response：

T is deduced as Int which is non-nullable

CodePudding user response：

that's because

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