scala self-type: member of type parameter error

问题

This is a followup to this question.

Why does this code not compile, and how do I fix it?

trait Vec[V] { self:V =>
  def -(v:V):V
  def dot(v:V):Double

  def norm:Double = math.sqrt(this dot this)
  def dist(v:V):Double = (this - v).norm
}

The error is:

Vec.scala:6: error: value norm is not a member of type parameter V
  def dist(v:V):V = (this - v).norm
                               ^

回答1:

By changing the definition of - to

def -(v:V):Vec[V]

回答2:

The proper solution is:

trait Vec[V <: Vec[V]] { self:V =>
  def -(v:V):V
  def dot(v:V):Double

  def norm:Double = math.sqrt(this dot this)
  def dist(v:V):Double = (this - v).norm
}

Props to Debilski for the answer to a related question.

来源：https://stackoverflow.com/questions/4774743/scala-self-type-member-of-type-parameter-error