问题在范围内.

一个简单的例子是

trait A { type T }
implicit val a: A { type T = Int } = null
def test(implicit x: A): Unit = {
  implicitly[x.T =:= Int] // doesn't compile, cannot prove that x.T =:= Int
}

您假设 x 是 a (在您的符号中， aIsBaseTc 是 doubleHoldsTypeMember ).但是实际上 x 不是 a ，当调用 test 时， x 将被解析(在的范围内> test 呼叫站点)，但在当前范围( test 定义范围)中定义了 a .同样， aIsBaseTc 不是 doubleHoldsTypeMember .

You assume that x is a (aIsBaseTc is doubleHoldsTypeMember in your notations). But actually x is not a, x will be resolved when test is called (in the scope of test call site) but a is defined in current scope (scope of test definition). Similarly, aIsBaseTc is not doubleHoldsTypeMember.

使用隐式解析时类型参数，为什么val放置很重要?(请参见 implicit x:X 和 implicitly [X] 之间的区别.)

When doing implicit resolution with type parameters, why does val placement matter? (See the difference between implicit x: X and implicitly[X].)

对于任何匿名课程

implicit val containerOfDoubleIsActual = new IsActualTC[Container[Double], Double] {
  def get(self: Self) = self.get // type mismatch; 
}

是

class IsActualTCImpl extends IsActualTC[Container[Double], Double] {
  def get(self: Self) = self.get // type mismatch; 
                                 // aIsBaseTc is not doubleHoldsTypeMember here
}
implicit val containerOfDoubleIsActual = 
  new IsActualTCImpl // implicit is resolved here
                     // aIsBaseTc becomes doubleHoldsTypeMember here

并且由于 aIsBaseTc 不是 doubleHoldsTypeMember ，所以 aIsBaseTc.Self 不是 Double .

And since aIsBaseTc is not doubleHoldsTypeMember, aIsBaseTc.Self is not Double.

可能的解决方法是在 IsActualTC

abstract class IsActualTC[A, T, S](implicit val aIsBaseTc: IsBaseTC[T] {type Self = S}) extends IsBaseTC {
  type Self = A
  def get(self: A): S
}

implicit val containerOfDoubleIsActual = new IsActualTC[Container[Double], Double, Double] {
  def get(self: Self) = self.get
}

或在 IsActualTC

abstract class IsActualTC[A, T](implicit val aIsBaseTc: IsBaseTC[T] {type Self = T}) extends IsBaseTC {
  type Self = A
  def get(self: A): aIsBaseTc.Self
}

implicit val containerOfDoubleIsActual = new IsActualTC[Container[Double], Double] {
  def get(self: Self) = self.get
}