ClassTag for type class parameter

Question

I want to create a type class that will constrain its defined abstract type to have a ClassTag . Here is a simplified example:

trait A[T] {
  type B <: Z

  val tag = implicitly[ClassTag[B]]
}

// Error:(8, 24) No ClassTag available for A.this.B
//  val tag = implicitly[ClassTag[B]]
                  ^

I need B to have a ClassTag[B] and I can't define A like trait A[T, B: ClassTag] because I want A be implicitly available for T , like in def foo[T: A](t: T) . B also has to be upper-bound to some Z , but it seems to make no difference.

Is there a way to express ClassTag constraint on B ?

Answer 1

There is no way for the compiler to provide you with a ClassTag here, as it has no clue what B might end up being.

Make it an abstract def instead, and let the concrete implementation of A provide it:

trait A[T] {
  type B <: Z
  def tag: ClassTag[B] // you may want to declare it as implicit
}

and, for instance,

new A[Int] {
  type B = Z
  def tag = implicitly
}