Function that takes an argument that must be a subclass of a specified class, and that subclass must implement a specified protocol

subclassAndProtocol.swift

protocol CanBark {
    func bark() -> String
}

class Animal {
    func animalType() -> String {
        return "Animal"
    }
}

class Dog: Animal, CanBark {
    override func animalType() -> String {
        return "Dog"
    }

    func bark() -> String {
        return "Woof"
    }
}

func makeAnimalBark<T where T: CanBark, T: Animal>(barkingAnimal: T) -> String {
    let animalType = barkingAnimal.animalType()
    // let bark = barkingAnimal.bark()   <-- Doesn't work. Why?
    let canBark = barkingAnimal as CanBark
    let bark = canBark.bark()
    return "\(animalType) says \"\(bark)\""
}

let dog = Dog()
makeAnimalBark(dog)              // Dog says "Woof"

I wrote this in response to https://twitter.com/joshaber/status/496833803160416256

It's weird that the let canBark = barkingAnimal as CanBark line is needed, but when I tried let bark = barkingAnimal.bark(), the compiler crashed. I posted a question to the dev forums: https://devforums.apple.com/thread/239594

FWIW, in a project let bark = barkingAnimal.bark() crashes the compiler 😿

Bug report filed: http://www.openradar.me/17929884