Expression of type doesn't conform to Expected type B

I have following code to understand covariant and lower bound, I intentionally make the code have compiling errors.

The getOrElse works, which is similar from the getOrElse method of Option[+T].

I would ask why getOrElse2 and getOrElse3 don't work to better understand covariant and lower bound.

The compiling error is pasted as comments in the code:

class  MyOption[+A](val x: A) {
  def get():A = x

  //similar to Option.getOrElse,which works
  def getOrElse[B>:A ](default: => B): B = {
    if (x != null) x else  default
  }

  //Compiling Error: Expression of type A doesn't conform to Expected type B
  def getOrElse2[B, A<:B ](default: => B): B = {
    if (x != null) x else  default
  }

  //Covariant type A occurs in controvariant position in type A of value B
  def getOrElse3[B <: A](default:=>B): A = {
    if (x != null) x else  default
  }
}