Type Predicates that just ain't working...

TypePredicate2

sealed trait TypeBoolean
sealed trait True extends TypeBoolean
sealed trait False extends TypeBoolean

trait Conditional{
  type Result <: TypeBoolean
}

abstract class TypePredicate2[A,B,Cond[_,_] <: Conditional,Implicit[_,_]] extends Conditional{
  implicit def pred[Child,Parent](implicit ev:Implicit[Child,Parent]):Cond[Child,Parent]

  abstract class Answer(implicit ev: Cond[A,B]) extends Conditional{
    type Result = Cond[A,B]#Result
  }

  type Result = Answer#Result
}

sealed abstract class SubType[A,B] extends Conditional
object IsSubtypeOf{
  implicit def default[Child,Parent] = new SubType[Child,Parent]{ type Result = False }
}

abstract class IsSubtypeOf[A,B] extends TypePredicate2[A,B,SubType,<:<]{
  implicit def pred[Child,Parent](implicit ev: <:<[Child,Parent]):SubType[Child,Parent] =
    new SubType[Child,Parent] with (Child => Parent){ def apply(x: Child):Parent = x
      type Result = True
    }
}

class Parent
class Child extends Parent
class Foo{
  def bar[A <: TypeBoolean](implicit ev: =:=[A,True]) = 1
  def ohNoes() = bar[IsSubtypeOf[Child,Parent]#Result]
}

[error] could not find implicit value for parameter ev: =:=[SubType[Child,Parent]#Result,True]
[error]   def ohNodes() = bar[IsSubtypeOf[Child,Parent]#Result]