@kiptoo your

Box<T>

is invariant, that means

Box<Int>

is not a subtype of

Box<Number>

https://proandroiddev.com/understanding-generics-and-variance-in-kotlin-714c14564c47

hi @Andreas Sinz i now get the general concept (i actually went thru the article you shared earlier, it's excellent). i can tell the compiler that my intention is for IntBox to be a subtype of Box<T> by doing:

open class Box<out T>(t: T) {}

, but what if some of my functions in Box need to take in parameters of type (or subtype) T, e.g

Box.createBox(box: Box<T>)

?

@kiptoo you can only have Contravariance or Covariance, meaning

T

only occurs in the in-position or out-position, not both at the same time, otherwise you could create code that fails at runtime

good point. looks like i'll have to settle for covariance in this case