what does `Foo!!` mean??

# announcements

[E.Kisaragi]

what does Foo!! mean??

[fatih]

You mean !! ?

[henrikhorbovyi]

You are telling to the compiler that Foo must not be null

[jbnizet]

not-null assertion operator: https://kotlinlang.org/docs/reference/null-safety.html#the--operator

[E.Kisaragi]

i meant type

[diesieben07]

There is no Foo!! type. You probably mean Foo!, which is a platform type.

[E.Kisaragi]

https://kotlin.github.io/kotlin-spec/
😇

[diesieben07]

I don't think that is actualyl implemented, but don't quote me on that. Would be useful!

[dmitriy.novozhilov]

T!! is a "definitely not null type for type parameter T" used in compiler
When you have some type parameter T without any upper bounds it can be nullable for some substitutions of T, so you can not call something like this:

fun Any.foo() {}

fun <T> test(x: T) {
    // x has type `T`
    x.foo() // error since T may be null
}

But if compiler knows that value of type T somehow can not be null (e.g. after smartcast) than it should describe it, but it can't remove ? from type, because T already don't have T. For such cases we have T!! which means intersection type T & Any

fun Any.foo() {}

fun <T> test(x: T) {
    if (x != null) {
        // x smartcasted to type `T!!`
        x.foo() // ok
    }
}

[E.Kisaragi]

So does 'T!!' stands for 'T & Any' ?

[dmitriy.novozhilov]

Yes. T!! introduced as separate kind of type because it's more easy and performant to use it instead of intersection type in such specific case

[E.Kisaragi]

thanks, your help was solved one of stucked my question 😄