protocol P {
    func foo()
}

extension P {
    func foo() { print("P") }
}

class A: P {}
class B: A {
    func foo() { print("B") } // <- Does not work.
}
class C: P {
    func foo() { print("C") }
}
class D: C {
    override func foo() { print("D") }
}
let instances: [P] = [A(), B(), C(), D()]
instances.forEach { $0.foo() }

/* Actual output:
 P
 P
 C
 D
 */

/* Desired output:
 P
 B
 C
 D
 */