class B
object C extends B

class F[T <: B](cons: => T) {
  def f = cons
}
class F2[T <: B](cons: => T) extends F(cons)

object D extends F2(C) // works
object E extends F2(new B {})

object Test {
  def main(args: Array[String]): Unit = {
    D.f
    E.f
  }
}