May be it should be explained in https://docs.scala-lang.org/tour/classes.html .
I think that we need seperate bold section for it 
I remember when I had started to work with scala the google told me to make something like
class A[T >: Null](someT: T) {
var x: T = null
val y: T = x
def init(): Unit = x = someT
}
In the end, I lost about two days to find out and fix that mistake in my final class hierarchy.
Yes, I have understood all these differences, but …,… ,…