class MyClass<T> {
    var prop: T = null // error
}

why does this not work? I thought that the upper bound of a type parameter is by default

Any?

I believe the issue you'll have here is that T could also be "Any" not just "Any?". For example:

val myThing = MyClass<String>()
val otherThing = MyClass<String?>()

What you might want to do is to use a non-nullable T (i.e.

class MyClass<T: Any>()

) and a nullable prop definition

thanks, I think I get it