puzzle.swift

// Example 1
protocol P {
  func f() -> Self
}

extension P {
  func f() -> Self {
    return self
  }
}

class C : P {}

// Example 2
protocol Q {
  associatedtype I = Self
  func f() -> I
}

extension Q where Self.I == Self {
  func f() -> Self {
    return self
  }
}

class D : Q {}

// Example 1 fails to type check in Swift 4.0.

// Example 2 works in Swift 4.0, but this doesn't work, which is counter-intuitive because E should inherit all members of D:
class E : D {}
E().f()

// b.swift:19:5: error: 'E' is not convertible to 'D'
// E().f()
// ~~~~^