Is it expected behavior I thought that `Surface` is subset o

Question

Is it expected behavior I thought that `Surface` is subset of `Surface ` so it will be expanded to it If unusual or should not happen I will try to make a minimal reproducible example out of <https github com streetcomplete StreetComplete pull 4642 pullrequestreview 1304594775>

Mateusz Konieczny · Answer

For now I created ```fun Surface asItem DisplayItem lt Surface gt = Item this iconResId titleResId ``` and ```fun Surface asItemWithFakeNullPossibility DisplayItem lt Surface gt = Item this iconResId titleResId ``` but I suspect there is a better way to handle this

Wout Werkman · Answer

If you did not specify that the generic parameter of `DisplayItem` is `out` then this is expected I think you want to add `out`

Mateusz Konieczny · Answer

I have not specified it I guess I need to learn what `out` is doing

mkrussel · Answer

`Surface` is a subtype of `Surface ` but that doesn t make `DisplayItem lt Surface gt ` a subtype of `DisplayItem lt Surface gt ` The subtyping of the generic types are based on the variance A good explanation <https typealias com guides illustrated guide covariance contravariance >

Mateusz Konieczny · Answer

Thanks for hints gt but that doesn t make `DisplayItem lt Surface gt ` a subtype of `DisplayItem lt Surface gt ` I see that I need to learn more I was expecting exact opposite and got confused what is going on

Mateusz Konieczny · Answer

If you have a better place to learn about `out` than official docs please let me know

mkrussel · Answer

I think typealias is the best place

ephemient · Answer

`Producer lt T gt ` is a subtype of `Producer lt T gt ` `Consumer lt T gt ` is a subtype of `Consumer lt T gt `

mkrussel · Answer

But the `out` and `in` keywords change the variance of the generics which changes the subtyping rules

ephemient · Answer

Kotlin uses the terms `in` and `out` because that s where it goes

ephemient · Answer

e g a consumer can take `in T` a producer gives `out T`

CLOVIS · Answer

New users of any language are often confused by variance because we tend to think of immutable data structures which are always covariant a `List lt Int gt ` is a `List lt Number gt ` but this is unsafe if the list can be modified

ephemient · Answer

for historical Java reasons arrays are covariant even though it s unsafe