Scala型证据

在scala源代码中我可以看到这段代码:

@implicitNotFound(msg = "Cannot prove that ${From} <:< ${To}.")
sealed abstract class <:<[-From, +To] extends (From => To) with Serializable
private[this] final val singleton_<:< = new <:<[Any,Any] { def apply(x: Any): Any = x }
// not in the <:< companion object because it is also
// intended to subsume identity (which is no longer implicit)
implicit def conforms[A]: A <:< A = singleton_<:<.asInstanceOf[A <:< A]

@implicitNotFound(msg = "Cannot prove that ${From} =:= ${To}.")
sealed abstract class =:=[From, To] extends (From => To) with Serializable
private[this] final val singleton_=:= = new =:=[Any,Any] { def apply(x: Any): Any = x }
object =:= {
    implicit def tpEquals[A]: A =:= A = singleton_=:=.asInstanceOf[A =:= A]
}

我并不完全清楚为什么这两个类都会扩展Function1.不会扩展任何东西吗？