It seems like it should be allowed, yet this fails...

# getting-started

[CLOVIS]

It seems like it should be allowed, yet this fails to compile:

interface Param

data class A<P : Param?>(val v: (P) -> Unit)

fun <P> create(v: (P) -> Unit) = A(v)

fun main() {
    val a = create { _: Nothing? -> }

    println(a.v(null)) // Null can not be a value of a non-null type Nothing
}

According to IDEA, a has type A<out Any?>, so why is null not allowed? (playground link)

[Sam]

out means that the type can only be used as a return/output type; in means that the type can only be used as a parameter/input type.

[Sam]

So it’s not possible to call a method where one of the input types is out projected

[Robert Williams]

You need to make the signature for create match the signature for A:

fun <P:  Param?> create

[Juliane Lehmann]

So that the type param for A will get passed on, and you end up with a being of type A<Nothing?> as you expect, instead of type A<out Any?>.

[Robert Williams]

Assuming Param is doing something useful this probably isn't what you want anyway because you can't call v(Param) on A<Nothing?> 🤔

[Robert Williams]

println(a.v(null as Param?))
    println(a.v(object: Param {}))

^^^compile errors^^^

[CLOVIS]

I see, thanks.