Scala Circe - Extending Decoder getting no implicit arguments

Question

I am still learning scala and attempting to use circe's decoders, but running into a little trouble with a context bound I think. I'm unsure why scala is expecting this implicit argument?

 abstract class Input[A](namedDecoder: String) extends Decoder[A]
  abstract class Test[T: Input](args: Int)

  case class InputImpl(hash: String, namedDecoder: String) extends Input(namedDecoder)
  class TestImpl(x: Int) extends Test[InputImpl](x)

I'm getting an error: No implicit arguments of type Input[InputImpl] and I'm a little confused what I'm missing here. Thanks for any help!