AnyDerivative implemented using a class. It's not possible because class computed properties cannot return Self :(

.swift

// Type-erased box.
fileprivate class AnyDerivativeBox : Differentiable & AdditiveArithmetic {
  public typealias TangentVector = AnyDerivativeBox
  public typealias CotangentVector = AnyDerivativeBox
  public typealias AllDifferentiableVariables = AnyDerivativeBox

  public static func == (lhs: AnyDerivativeBox, rhs: AnyDerivativeBox) -> Bool {
    fatalError("Must override")
  }
  public static var zero: Self {
    fatalError("Must override")
  }
  public static func + (lhs: AnyDerivativeBox, rhs: AnyDerivativeBox) -> Self {
    fatalError("Must override")
  }
  public static func += (lhs: inout AnyDerivativeBox, rhs: AnyDerivativeBox) {
    fatalError("Must override")
  }
  public static func - (lhs: AnyDerivativeBox, rhs: AnyDerivativeBox) -> Self {
    fatalError("Must override")
  }
  public static func -= (lhs: inout AnyDerivativeBox, rhs: AnyDerivativeBox) {
    fatalError("Must override")
  }
  public var allDifferentiableVariables: AnyDerivativeBox {
    get {
      fatalError("Must override")
    }
    set {
      fatalError("Must override")
    }
  }
  func moved(along direction: TangentVector) -> Self {
    fatalError("Must override")
  }
  func tangentVector(from direction: CotangentVector) -> TangentVector {
    fatalError("Must override")
  }
}

/*
fileprivate class ConcreteDerivativeBox<Derivative : Differentiable & AdditiveArithmetic> : AnyDerivative {
  private var wrapped: Derivative

  public class func + (lhs: AnyDerivativeBox, rhs: AnyDerivativeBox) -> Self {
    precondition(type(of: lhs) == type(of: rhs), "Type mismatch")
    
  }
  public class func - (lhs: AnyDerivativeBox, rhs: AnyDerivativeBox) -> Self {
    precondition(type(of: lhs) == type(of: rhs), "Type mismatch")

  }
  public var allDifferentiableVariables: AnyDerivative {
    get {
      fatalError("Must override")
    }
  }
  func moved(along direction: TangentVector) -> AnyDerivative {
    fatalError("Must override")
  }
  func tangentVector(from direction: CotangentVector) -> TangentVector {
    fatalError("Must override")
  }
}

// Concrete box.
fileprivate func makeAnyDerivativeBox<T : Differentiable & AdditiveArithmetic>(_ value: T) -> AnyDerivativeBox {
  return ConcreteDerivativeBox(value)
}
*/

/*
// Erasure type.
public struct AnyDerivative : Differentiable {
  private let box: AnyDerivativeBox

  public typealias TangentVector = AnyDerivative
  public typealias CotangentVector = AnyDerivative
  public typealias AllDifferentiableVariables = AnyDerivative

  public func + (lhs: AnyDerivativeBox, rhs: AnyDerivativeBox) -> AnyDerivativeBox {
    
  }
  public func - (lhs: AnyDerivativeBox, rhs: AnyDerivativeBox) -> AnyDerivativeBox {

  }
  public var allDifferentiableVariables: AnyDerivative {
    get {
      fatalError("Must override")
    }
  }
  func moved(along direction: TangentVector) -> AnyDerivative {
    fatalError("Must override")
  }
  func tangentVector(from direction: CotangentVector) -> TangentVector {
    fatalError("Must override")
  }
}
*/