ProxyEvidence.scala

trait Foo { def i: Int }
trait Foo2[T] { def sq(t: T): Int }

trait Bar[T] {
  def cube(t: T): Int
}

object Bar {
  implicit def foo[T <: Foo: Foo2]: Bar[T] = {
    val ctx = implicitly[Foo2[T]]
    new Bar[T] { def cube(t: T) = t.i * ctx.sq(t) }
  }
}

case class Adu(i: Int) extends Foo

def use[T <: Foo: Foo2](t: T) = true 
def use2[T: Bar](t: T) = true 

implicit val aduIsFoo2 = new Foo2[Adu] { def sq(t: Adu) = t.i * t.i }
use(Adu(2)) // OK!
use2(Adu(4)) // OK!