gistfile1.scala

scala> sealed abstract class A

case object B extends A
case object C extends A

def hoge(xs:Map[A,Int]){ xs foreach println }

val list = Map(B -> 1 ,C -> 2)
defined class A
defined module B
defined module C
hoge: (xs: Map[A,Int])Unit
list: scala.collection.immutable.Map[Product with Serializable with A,Int] = Map(B -> 1, C -> 2)

scala> hoge(list)
<console>:27: error: type mismatch;
 found   : scala.collection.immutable.Map[Product with Serializable with A,Int]
 required: Map[A,Int]
Note: Product with Serializable with A <: A, but trait Map is invariant in type A.
You may wish to investigate a wildcard type such as `_ <: A`. (SLS 3.2.10)
       hoge(list)
            ^