Type mismatch with generics sum type

Question

I have a sum trait, that looks like as follow:

sealed trait Sum[+A, +B]

final case class Failure[A](value: A) extends Sum[A, Nothing]

final case class Success[B](value: B) extends Sum[Nothing, B]

When I try to create a new variable as:

val s1: Sum[Int, Nothing] = Success(4)

I've got following error:

Error:(5, 41) type mismatch;
 found   : Int(4)
 required: Nothing
    val s1: Sum[Int, Nothing] = Success(4)

Why?

And why this is working:

val s1: Sum[Int, Int] = Success(4)

Answer 1

因为B是第二个类型参数，而不是第一个：

val s1: Sum[Nothing, Int] = Success(4)