I am not sure, it looks to me that it is some kind of bug or bad implementation with protocol extensions in Swift 2.0.
I have protocolA, protocolB extending protoco
Interesting. Looks like a bug to me. It does recognize functions on a class, but not on extension. No matter what type Instance has. Moreover without an extension code would not be compilable, since protocol methods are non optional. So really looks like a bug/feature? in responds to selector implementation.