为什么有部分功能

val warpedEngineers: PartialFunction[Warped[Crewmember],Warped[Engineer]] = {
  case v@Warped(Engineer(name: String)) => v.asInstanceOf[Warped[Engineer]]
}

似乎要求在RHS上投放asInstanceOf，而以下内容则不需要

val engineers: PartialFunction[Crewmember,Engineer] = {
  case v@Engineer(name) => v
}

given

sealed trait Crewmember
case class Engineer(name: String) extends Crewmember
case class Commander(name: String) extends Crewmember

case class Warped[+A <: Crewmember](v: A)

val engineers: PartialFunction[Crewmember,Engineer] = {
  case v@Engineer(name) => v
}

val warpedEngineers: PartialFunction[Warped[Crewmember],Warped[Engineer]] = {
  case v@Warped(Engineer(name: String)) => v.asInstanceOf[Warped[Engineer]]
}

val crew: List[Crewmember] = 
  List(Engineer("Geordi"),Commander("Picard"),Engineer("Scott"),Commander("Kirk"))

val warpedCrew: List[Warped[Crewmember]] = 
  List(Warped(Engineer("Geordi")),Warped(Commander("Picard")),Warped(Engineer("Scott")),Warped(Commander("Kirk")))

crew collect engineers
// res0: List[Engineer] = List(Engineer(Geordi),Engineer(Scott))

warpedCrew collect warpedEngineers
// res1: List[Warped[Engineer]] = List(Warped(Engineer(Geordi)),Warped(Engineer(Scott)))

像这样可以避免使用asInstanceOf进行铸造

case Warped(eng: Engineer) => Warped(eng) 

但是我想知道为什么编译器不插入隐式asInstanceOf而是将v键入Warped[Crewmember]

val warpedEngineers: PartialFunction[Warped[Crewmember],Warped[Engineer]] = {
  case v@Warped(Engineer(name: String)) => v 
}

Error: type mismatch;
 found   : Warped[Crewmember]
 required: Warped[Engineer]
      case v@Warped(Engineer(name: String)) => v

根据SLS 8.1.3: Pattern Binders

模式绑定器?@?由模式变量?和模式组成 ?。变量type的类型是由隐含的静态类型?。 模式?。该模式与该模式匹配的任何值?匹配 ?，它将变量名绑定到该值。

如果模式matches仅匹配的值，则模式?表示类型?。 ?类型。