喷Json`RootJsonFormat`的不变性？

Question

鉴于以下内容：

scala> trait Parent
defined trait Parent

scala> case class Boy(name: String) extends Parent
defined class Boy

scala> case class Girl(id: Int) extends Parent
defined class Girl

我尝试为它们定义一个隐式格式化程序（反序列化和序列化）：

scala> object Formats extends DefaultJsonProtocol { 
     |   implicit val format: RootJsonFormat[Parent] = jsonFormat1(Boy.apply)
     |   implicit val girlFormat: RootJsonFormat[Parent] = jsonFormat1(Girl.apply)
     | }

但是我遇到了编译时错误：

<console>:22: error: type mismatch;
 found   : spray.json.RootJsonFormat[Boy]
 required: spray.json.RootJsonFormat[Parent]
Note: Boy <: Parent, but trait RootJsonFormat is invariant in type T.
You may wish to define T as +T instead. (SLS 4.5)
         implicit val format: RootJsonFormat[Parent] = jsonFormat1(Boy.apply)
                                                                  ^
<console>:23: error: type mismatch;
 found   : spray.json.RootJsonFormat[Girl]
 required: spray.json.RootJsonFormat[Parent]
Note: Girl <: Parent, but trait RootJsonFormat is invariant in type T.
You may wish to define T as +T instead. (SLS 4.5)
         implicit val girlFormat: RootJsonFormat[Parent] = jsonFormat1(Girl.apply)
                                                                      ^

据我了解这个编译时错误消息，RootJsonFormat是不变的是它的类型参数。

为什么？