Scala override method with subclass as parameter type

I have a trait A which has a method def fun1( b:B ):C I want A's subclass implement B with more detailed type:

Here is the code:

trait B
trait C

trait A {
  def fun1( b:B ):C
}

class B1 extends B{
}

class B2 extends B{
}


class C1 extends C{
}

class C2 extends C{
}

I hope A's subclass can be declared as below

class X1 extends A{
    override def fun1(b:B1):C1 = ...
}


class X2 extends A{
    override def fun1(b:B2):C2 = ...
}

However, the compiler will complain that X1 overrides nothing. I have to manually match detailed B's type, like below.

class X1 extends A{
    override def fun1(b:B):C = b match {case x:B1 => ... }
}


class X2 extends A{
    override def fun1(b:B2):C2 = b match {case x:B2 => ... }
}

This method cannot check the correct type during compiling. How can I achieve the first implementation? Is there any design pattern to handle this problem?

A similar question is C# Override method with subclass parameter