为什么scala无法对f绑定多态进行类型推断？(Why scala fails type inference for f-bound polymorphism?)

 举例说明问题： 
 
trait WTF[W <: WTF[W]] {
  def get : Int
}

trait Zero extends WTF[Zero] {
  override def get : Int = 0
}
case object Zero extends Zero

final case class Box(inner : Int) extends WTF[Box] {
  override def get : Int = inner
}

def printWTF[W <: WTF[W]](w : W) = println(w.get)

printWTF(Box(-1))
printWTF(Zero)
 
 Box ，但Zero产生错误： 
 
WTF.scala:22: error: inferred type arguments [Zero.type] do not conform to method printWTF's type parameter bounds [W <: WTF[W]]
  printWTF(Zero)
  ^
WTF.scala:22: error: type mismatch;
 found   : Zero.type
 required: W
  printWTF(Zero)
           ^
two errors found
 
 如果我手动注释类型，它编译： 
 
printWTF[Zero](Zero)
printWTF(Zero : Zero)
 
 第一行按预期工作。 我经常遇到无法从参数推断出类型参数的情况。 例如def test[A](x : Int) : Unit 。 A类型在参数签名中没有出现，因此您应该手动指定它。 
 
 但后者对我来说非常模糊。 我刚刚添加了类型转换，它始终是真的，并且奇迹般地编译器学习如何推断方法类型参数。 但Zero始终是Zero类型，为什么编译器无法在没有提示的情况下推断它呢？ 

Simple example to illustrate the issue:
 
trait WTF[W <: WTF[W]] {
  def get : Int
}

trait Zero extends WTF[Zero] {
  override def get : Int = 0
}
case object Zero extends Zero

final case class Box(inner : Int) extends WTF[Box] {
  override def get : Int = inner
}

def printWTF[W <: WTF[W]](w : W) = println(w.get)

printWTF(Box(-1))
printWTF(Zero)
 
Box is ok, but Zero produces error:
 
WTF.scala:22: error: inferred type arguments [Zero.type] do not conform to method printWTF's type parameter bounds [W <: WTF[W]]
  printWTF(Zero)
  ^
WTF.scala:22: error: type mismatch;
 found   : Zero.type
 required: W
  printWTF(Zero)
           ^
two errors found
 
If I annotate type manually, it compiles:
 
printWTF[Zero](Zero)
printWTF(Zero : Zero)
 
The first line works as expected. I frequently encounter cases where type parameters could not be inferred from arguments. e.g. def test[A](x : Int) : Unit. The A type appears nowhere in the argument signature, so you should specify it manually.
 
But the latter is very obscured to me. I just added type cast that always is true, and miraculously the compiler learns how to infer method type parameters. But Zero is always of Zero type, why the compiler could not infer it without hints from me?

原文：https://stackoverflow.com/questions/42181021