scala> sealed trait Gender
defined trait Gender
scala> case object Male extends Gender
defined module Male
scala> case object Female extends Gender
defined module Female
scala> Map(Male -> Male, Female -> Female, Male -> Female, Female -> Male)
res2: scala.collection.immutable.Map[Product with Gender,Product with Gender] =
Map((Male,Female), (Female,Male))
为什么在上面的代码中,类型res2是？Map[Product with Gender, Product with Gender]而不是Map[Gender, Gender]？为什么我提供给地图的四个条目,只添加了两个？