Decoding subclasses

I'm trying to convert some code to use Argo, and I'm getting stuck where the existing code uses subclasses. I can't get type inference to pass.

If anyone knows how to get this to work, I'd love some help.

Error Messages

error: cannot convert value of type 'Decoded<_>' to specified type 'Decoded<Child>'
    let child: Decoded<Child> = json <| "child"
                                ~~~~~^~~~~~~~~~
error: cannot convert value of type 'Decoded<[_]?>' (aka 'Decoded<Optional<Array<_>>>') to specified type 'Decoded<[Child]?>' (aka 'Decoded<Optional<Array<Child>>>')
    let children: Decoded<[Child]?> = json <||? "children"
                                      ~~~~~^~~~~~~~~~~~~~~

Models

class Parent: Decodable {
  let title: String

  init(title: String) {
    self.title = title
  }

  typealias DecodedType = Parent
  static func decode(_ json: JSON) -> Decoded<Parent> {
    return curry(Parent.init)
      <^> json <| "title"
  }
}

class Child: Parent {
  let subtitle: String

  init(title: String, subtitle: String) {
    self.subtitle = subtitle
    super.init(title: title)
  }

  typealias DecodedType = Child
  static func decode(_ json: JSON) -> Decoded<Child> {
    return curry(Child.init)
      <^> json <| "title"
      <*> json <| "subtitle"
  }
}

final class Consumer: Decodable {
  let child: Child
  let children: [Child]?

  init(child: Child, children: [Child]?) {
    self.child = child
    self.children = children
  }

  static func decode(_ json: JSON) -> Decoded<Consumer> {
    // Changing these from Child to Parent makes it work
    let child: Decoded<Child> = json <| "child"
    let children: Decoded<[Child]?> = json <||? "children"
    return curry(self.init)
      <^> child
      <*> children
  }
}

Argo Version

Argo 4.1.2 Xcode 8.3.2

Dependency Manager

Carthage