Issue Description:

Let's start with a couple of sample protocols:

protocol Clonable {
func copy(toDepth depth: Int) -> Self?
}
protocol StronglyClonable: Clonable {
func copy(toDepth depth: Int) -> Self!
}

And create two sample classes in the form of:

class SampleX { init() {} }

When I make one class conform to {Clonable} and the second to {StronglyClonable}, I get the following suggested stubs (I filled in some sample implementations.):

class Sample1: Clonable {
func copy(toDepth depth: Int) -> Self? {
return Sample1() as? Self
}
init() {}
}

class Sample2: StronglyClonable {
func copy(toDepth depth: Int) -> Self! {
return (Sample2() as! Self)
}
func copy(toDepth depth: Int) -> Self? {
return Sample2() as? Self
}
init() {}
}

In the second class, the derived protocol's update to the method is treated as a new method, not one that overrides the method from its base protocol. Further, the playground suggests both versions of the method. Then the playground also flags the second occurrence as an invalid redeclaration. I don't know which parts of this paragraph are the bug.