Hey, I was messing around with covariance and cont...

# announcements

[Conor Fennell]

Hey,
I was messing around with covariance and contravariance and did not expect this to work.

It looks like I created a function which is bivariant. How is that possible as the following code compiles and runs.

class Container<in T>(private val t: T) 

open class Noun
open class City: Noun()
class Capital: City()

fun goCity(container: Container<City>) {
    println(container)
}

fun main() {
    goCity(Container(Noun())) // pass in a super type of City, this is what I would expect as it is defined contravariance
    goCity(Container(Capital())) // pass in a sub type of City, would not have expected this as it is defined contravariance
}

[Marc Knaup]

goCity(Container(Capital()))

This creates a Container<City> implicitly because it satisfies both:
passing in Capital as a constructor parameter
and the expected type Container<City> for goCity

[Conor Fennell]

🙇 Thank you