RHS上的模式绑定变量的类型与LHS上的匹配模式不对应

Question

为什么执行部分功能

case v@Warped(Engineer(name: String)) => v.asInstanceOf[Warped[Engineer]]

当将模式变量asInstanceOf绑定到给定的静态类型v的模式时，似乎要求在RHS上强制转换为Warped[Engineer]

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

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

warpedCrew collect warpedEngineers
// res6: List[Warped[Engineer]] = List(Warped(Engineer(Geordi)), Warped(Engineer(Scott)))

可以像这样避免使用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𝑥@𝑝由模式变量𝑥和模式组成𝑝。变量type的类型是由隐含的静态类型𝑇。模式𝑝。该模式与该模式匹配的任何值𝑣匹配𝑝，它将变量名绑定到该值。

如果模式仅与的值匹配，则模式𝑝表示类型𝑇类型𝑇。

Answer 1

[Warped(Engineer(name)) in left in