gistfile1.txt

protocol Foo1 {
    func foo1() -> String
}

protocol Foo2 {
    func foo2() -> String
}

extension Foo1 where Self : Foo2 {
    func foo1() -> String {
        return "foo" + self.foo2()
    }
}

struct Foo : Foo1, Foo2 {
    func foo2() -> String {
        return "test"
    }
    
    func test() {
        print(self.foo1())
    }
}

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        print(Foo().test())
    }
}