extension SomeViewController {
    func foo() -> Bool {
        return false
    }
}

class SomeViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
    }
}

class SomeViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
    }
}

extension SomeViewController {
    func foo() -> Bool {
        return false
    }
}

view controller does not have an outlet named (subview)

// This caused the (!) problem in Interface Builder

#if( MAC_OS_X_VERSION_MAX_ALLOWED > MAC_OS_X_VERSION_10_10 )
@interface CLChatWindowController : NSObject <WebPolicyDelegate, WebUIDelegate, WebFrameLoadDelegate>
#else
@interface CLChatWindowController : NSObject
#endif
...


// Removing all the Preprocessor code fixed the problem

@interface CLChatWindowController : NSObject <WebPolicyDelegate, WebUIDelegate, WebFrameLoadDelegate>
....