scala – 为什么隐式搜索会受到不相关的类型参数的影响？

object UnitsEx {
  case class Quantity[M <: MInt,T: Numeric](value: T) {
    private val num = implicitly[Numeric[T]]
    def *[M2 <: MInt](m: Quantity[M2,T]) = 
      Quantity[M,T](num.times(value,m.value))
  }

  implicit def measure[T: Numeric](v: T): Quantity[_0,T] = Quantity[_0,T](v)
  implicit def numericToQuantity[T: Numeric](v: T): QuantityConstructor[T] = 
    new QuantityConstructor[T](v)

  class QuantityConstructor[T: Numeric](v: T) {
    def m = Quantity[_1,T](v)
  }

  sealed trait MInt
  final class _0 extends MInt
  final class _1 extends MInt
}

此代码段显示了我目前使用的用法和编译器错误：

import UnitsEx._

(1 m) * 1 // Works
1 * (1 m) // Doesn't work:
/*
<console>:1: error: overloaded method value * with alternatives:
(x: Double)Double <and>
(x: Float)Float <and>
(x: Long)Long <and>
(x: Int)Int <and>
(x: Char)Int <and>
(x: Short)Int <and>
(x: Byte)Int
cannot be applied to (UnitsEx.Quantity[UnitsEx._1,Int])
1 * (1 m)
^
*/

用度量包装1将解决问题,但为什么不应用隐含的范围？

如果我删除下一个片段中的类型参数M,它就会开始工作,虽然我看不到该类型参数与隐式搜索的关系：

object UnitsEx2 {   
  case class Quantity[T: Numeric](value: T) {
    private val num = implicitly[Numeric[T]]
    def *(m: Quantity[T]) = Quantity[T](num.times(value,m.value))
  }

  implicit def measure[T: Numeric](v: T): Quantity[T] = Quantity[T](v)
  implicit def numericToQuantity[T: Numeric](v: T): QuantityConstructor[T] = 
    new QuantityConstructor[T](v)

  class QuantityConstructor[T: Numeric](v: T) {
    def m = Quantity[T](v)
  }
}

这是预期的还是类型检查器的已知限制？