How to specialize on a type projection in Scala?

Question

Statement of the question

Consider a type T that contains an abstract type member A:

trait T {
  type A
}
         


        

Answer 1

This is a compiler limitation; one cannot generally specialize on elements of a type parameter. However, the proposed trick is good enough for my purposes:

trait Types {
  type A
  type B
}

trait GenOps[@specialized A, @specialized B] {
  ...
}

trait Ops[T <: Types] extends GenOps[T#A, T#B]

This way the trait Ops gets specialized because it inherits the specialized implementations in trait GenOps. My motivation is that I want trait Ops to take a single type parameter T, rather than both T#A and T#B (this becomes necessary when Ops also takes a higher kinded type that expects T as a parameter).

Answer 2

I can't see how that could possibly work. Specialization is done when compiling the class, and, at that time, A isn't known.