Scala8079.scala

  trait TestCo {
    type CoVariant[+T, _ <: Exception]
    def unwrap[T](value: => CoVariant[T, _ <: Exception]): T
  }
// works fine, since Option is covariant
  class TestCoImpl extends TestCo {
    type CoVariant[+T, _ <: Exception] = Option[T]
    def unwrap[T](value: => CoVariant[T, _ <: Exception]): T = value.get
  }
//covariant type CoVariant occurs in contravariant position in type => TestCoFailed.this.CoVariant[T] of value value
//    def unwrap[T](value: => CoVariant[T]): T = value
  class TestCoFailed extends TestCo {
    type CoVariant[+T, _ <: Exception] = T
    def unwrap[T](value: => CoVariant[T, _ <: Exception]): T = value
  }