View difference between Paste ID: UCrAdCgd and 580sQ2uh

 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
  */