is duplicated by:

• SR-4744 @objc Attribute validation should support Protocol Extension declaration

relates to:

• SR-802 Cannot implement objc protocol extension method
• SR-7301 Bad fixit suggested for protocol extension implementation for inherited objc protocol

Issue Description:

Given a swift protocol, extensions can be made conditionally upon Self being a specific type. That specific type can be declared as NSObject. So it stands to reason that the methods declared in the extension (where Self is a derivative of NSObject) should be isomorphic to extensions on the type itself. Extensions on NSObject derivatives can contain @objc declarations. Conditional protocol extensions however are specifically disallowed from this pattern (which seems inconsistent).

import Foundation

public protocol Bar {
    func baz()
}


extension Bar where Self: NSObject {
    @objc(foo) // <- @objc can only be used with members of classes, @objc protocols, and concrete extensions of classes
    func foo() {
        baz()
    }
}

class MyObject : NSObject, Bar {
    func baz() { }
}

let m = MyObject()
m.foo()

The restriction limits the capability of making interoperation between swift protocols and objective-c adopters that might be passed back into objc layers; which might prefer some mechanism for maintaining that interoperability. Abjectly disallowing this prevents this interoperability and impedes more appropriate swift behavior.