Implicit conversion to Functor

Hello,

I'm trying to convert an arbitrary type to a Functor using implicit conversions (see code below). it sounded like an interesting idea at the time but it absolutely doesn't work.

trait Functor[A, T[A]] {
  def fmap[B](f:A => B):T[B]
}

sealed case class Tree[A]() {
 
  override def toString = {
      this match {
        case Leaf(x) => x.toString
        case Branch(a,b) => "[" + a + "," + b + "]"
        case _ => "Fck" // Compiler complains that match is non exhaustive because
                      // "Tree[A] with Functor[A, Tree]" (see implicit conversion below)
                      // generates anonymous class Tree$$anon that extends Tree[A]
      }   
    }
  }

case class Leaf[A](a:A) extends Tree[A]
case class Branch[A](a:Tree[A], b:Tree[A]) extends Tree[A]

object Tree {
  implicit def treeAsFunctor[A](t:Tree[A]):Tree[A] with Functor[A, Tree] = {
    new Tree[A] with Functor[A, Tree] {
      def fmap[B](f:A => B):Tree[B] = {
        this match { // Line 27
          case Leaf(x) => Leaf(f(x))
          case Branch(a,b) => Branch(a fmap f, b fmap f)
          // Here the compiler does not complain that a case is missing, but it causes
          // the match error (compiler bug?)
        }
      }   
    }
  }
}

object FunctorTest {
  def main(args : Array[String]) : Unit = {
    val t = Branch(Leaf(0), Branch(Leaf(1), Leaf(2)))
    println(t)
    // [0,[1,2]]
    println(t fmap(_+1))
    // Exception in thread "main" scala.MatchError: Fck
    // at com.Tree$$anon$1.fmap(Functor.scala:27)
    // at com.Tree$$anon$1.fmap(Functor.scala:25)
    // at com.FunctorTest$.main(Functor.scala:41)
    // at com.FunctorTest.main(Functor.scala)
  }
}

Can someone explain me the match error and/or provide some suggestions to fix this code?

Thanks,
Sebastien