Hi, in the section on Declaration-site variance here <https://kotlinlang.org/docs/reference/generics...

# getting-started

[Mattias Flodin]

Hi, in the section on Declaration-site variance here https://kotlinlang.org/docs/reference/generics.html#declaration-site-variance there is an example

interface Source<out T> {
    fun nextT(): T
}

fun demo(strs: Source<String>) {
    val objects: Source<Any> = strs // This is OK, since T is an out-parameter
    // ...
}

If I replace String by a type argument T, it no longer compiles and says "Type mismatch: inferred type is Source<T> but Source<Any> was expected". Why is that? I had expected it work just like in the example.

[nanodeath]

guessing, but it's probably not guaranteed to be the same T

[Mattias Flodin]

For reference, this is the code I'm trying to compile.

interface Source<out T> {
    fun nextT(): T
}

fun <T> demo(strs: Source<T>) {
    val objects: Source<Any> = strs // Type mismatch: inferred type is Source<T> but Source<Any> was expected
}

As far as I can see there is only one T in demo(), and so it doesn't need to be "the same" as anything else?

[Mattias Flodin]

regardless of what T is, it should be possible to upcast Source<T> to Source<Any>.

[araqnid]

Any isn’t the ultimate supertype, Any? is

[araqnid]

If T is non-nullable, you should specify <out T : Any>

[araqnid]

by contrast, String is a subtype of Any, so it’s consistent that that would work

[araqnid]

or you can write Source<Any?> if that’s what you meant

[Mattias Flodin]

Oh alright, that makes a lot of sense. Thanks.

[thanksforallthefish]

this is my favourite source of generics compile errors, I am basically ignoring Any? at least once a week and still think Any is the ultimate supertype when writing, just to have the compiler reminding me I once again forgot about nulls

[Mattias Flodin]

If the compiler was slightly better at reading my mind it could probably have produced an error message that made my error obvious to me. E.g. "Type mismatch: inferred type is Source<T: Any?> but Source<Any> was expected"

[thanksforallthefish]

I heard Rust has a pretty good compiler when it comes to error notification. maybe the next area of improvement for kotlin could be to look at the Rust compiler and take some inspiration