为什么上课<；：&书信电报；[-From，+；To]需要在scala标准库中扩展（From=>；To）？

我正在研究scala的类型级编程，并对它有了一些了解。但我不知道为什么要这样做），我在REPL中编写了以下代码

trait <:<[-T, +U] // just 'plain' generic trait

// an implicit object will looked up by compiler
implicit def implicitAgent[A]: <:<[A, A] = new <:<[A,A] {}

def myFunc[T,U](one:T, two:U)(implicit ev: T <:< U): Unit = {
    println(one, two)

class Base {
    override def toString: String = "base"
}

class Derived extends Base {
    override def toString: String = "Derived"
}

myFunc(new Derived, new Base)

---

所以我的问题是类是什么，因为这样

隐式ev:T
(Derived,base)

scala> trait Foo
defined trait Foo

scala> def myFunc[T](t: T)(implicit ev: T <:< Foo): Foo = t
myFunc: [T](t: T)(implicit ev: T <:< Foo)Foo

scala> trait <:<[-T, +U]
defined trait $less$colon$less

scala> implicit def implicitAgent[A]: <:<[A, A] = new <:<[A,A] {}
implicitAgent: [A]=> A <:< A

scala> def myFunc[T](t: T)(implicit ev: T <:< Foo): Foo = t
<console>:14: error: type mismatch;
 found   : T
 required: Foo
       def myFunc[T](t: T)(implicit ev: T <:< Foo): Foo = t
                                                          ^