fun-with-self.swift

func foo<T>(_: T) {
  print("Self = \(T.self)")
}

protocol P {
  func requirement()
}

extension P {
  func requirement() {
    foo(self)
  }

  func extensionMethod() {
    foo(self)
  }
}

class Base : P {
  func classMethod() {
    print("- Class method calls requirement")
    requirement()
    print("- Class method calls extension method")
    extensionMethod()
  }

  func classMethodReturnsSelf() -> Self {
    print("- Class method returning Self calls requirement")
    requirement()
    print("- Class method returning Self calls extension method")
    extensionMethod()
    return self
  }
}

class Derived : Base {}

let d = Derived()

print("\n== Non-Self-returning class methods\n")

print("Calling class method:")
d.classMethod()
print("\nCalling class method with upcast value:")
(d as Base).classMethod()

print("\n== Self-returning class methods\n")

print("Calling class method returning self:")
d.classMethodReturnsSelf()
print("\nCalling class method returning self with upcast value:")
(d as Base).classMethodReturnsSelf()

print("\n== Protocol requirement\n")

print("Calling requirement:")
d.requirement()
print("\nCalling requirement with upcast value:")
(d as Base).requirement()
print("\nCalling requirement with existential value:")
(d as P).requirement()
print("^----- !!!")

print("\n== Extension method\n")

print("Calling extension method:")
d.extensionMethod()
print("\nCalling extension method with upcast value:")
(d as Base).extensionMethod()
print("\nCalling extension method with existential value:")
(d as P).extensionMethod()