Why is this cyclic reference with a type projection illegal?

Question

The following pseudo-Scala yields an \"illegal cyclic reference\" error:

trait GenT[A]
trait T extends GenT[T#A] {
  type A
}

Quest

Answer 1

In your initial example you can break the cycle by introducing an auxiliary type inbetween GenT[A] and T,

trait GenT[A]
trait TAux { type A }
trait T extends TAux with GenT[TAux#A]

But from your motivating example I don't think you need to go down this route. The constraint you're after can be expressed directly using a refinement,

trait T { type A }
def foo[A0, S1 <: T { type A = A0 }, S2 <: T { type A = A0 }] ...

Also bear in mind that you can surface a type member as a type parameter via a type alias,

trait T { type A }
type TParam[A0] = T { type A = A0 }
def foo[A0, S1 <: TParam[A0], S2 <: TParam[A0]] ...