Using 'Protocol' as a concrete type conforming to protocol 'Protocol' is not supported

Question

I have the following swift code:

protocol Animal {
var name: String { get }
}

struct Bird: Animal {
    var name: String
    var canEat: [Animal]
}

struct Mammal: Animal {
    var name: String
}

extension Array where Element: Animal {
    func mammalsEatenByBirds() -> [Mammal] {
        var eatenMammals: [Mammal] = []
        self.forEach { animal in
            if let bird = animal as? Bird {
                bird.canEat.forEach { eatenAnimal in
                    if let eatenMammal = eatenAnimal as? Mammal {
                        eatenMammals.append(eatenMammal)
                    } else if let eatenBird = eatenAnimal as? Bird {
                        let innerMammals = eatenBird.canEat.mammalsEatenByBirds()
                        eatenMammals.append(contentsOf: innerMammals)
                    }
                }
            }
        }
        return eatenMammals
    }
}

The compiler does not let me compile complaining: Using 'Animal' as a concrete type conforming to protocol 'Animal' is not supported at the point where I recursively call the function mammalsEatenByBirds()

I have seen some other answers but could not relate my problem to any of those.

Answer 1

Fix is replacing Element: Animal with Element == Animal.