Swift Generic Downcast Question

GenericDowncast.swift

protocol BaseProtocol {
  func doIt()
}
protocol DerivedProtocol : BaseProtocol {
  func hello()
}

class MyObject: DerivedProtocol {
  func doIt()  { print("doIt")  }
  func hello() { print("hello") }
}
let o = MyObject()

// BTW: I do not want `func workOnDerived(value: DerivedProtocol)` here,
//      this is supposed to receive the full type `T` from workOnBase.
func workOnDerived<T: DerivedProtocol>(value: T) {
  value.hello()
}

func workOnBase<T: BaseProtocol>(value: T) {
  // the compiler still has the full type T here,
  // i.e. T is `MyObject`
  if value is DerivedProtocol {
    // how can I call workOnDerived(value)?
    // or in other words, how to I downcast the protocol
    // requirement?
    workOnDerived(value: value) // how??
  }
  else { fatalError("unexpected") }
}

workOnBase(value: o) // entry point, cannot control this