Adding stricter cons and contain operators to covariant lists via an implicit class, take 2

gistfile1.scala

scala> implicit class BVList[A](xs: List[A]) {
     |   def :=:[B >: A](x: B)(implicit ev: B =:= A): List[B] = x :: xs
     |   def containz(elem: A): Boolean = xs.contains(elem)
     | }
defined class BVList

scala> val xs = List(1, 2, 3)
xs: List[Int] = List(1, 2, 3)

scala> 5.0 :: xs
res4: List[AnyVal] = List(5.0, 1, 2, 3)

scala> 5.0 :=: xs
<console>:19: error: Cannot prove that AnyVal =:= Int.
              5.0 :=: xs
                  ^

scala> 5 :=: xs
res6: List[Int] = List(5, 1, 2, 3)

scala> xs contains "bob"
res7: Boolean = false

scala> xs containz "bob"
<console>:19: error: type mismatch;
 found   : String("bob")
 required: Int
              xs containz "bob"
                          ^

scala> xs containz 3
res9: Boolean = true