[swift-evolution] [Mini-proposal] Require @nonobjc on members of @objc protocol extensions

[Douglas Gregor]

> On Jan 4, 2016, at 8:54 PM, Kevin Lundberg <kevin at klundberg.com> wrote:
> 
> I like this idea, but I would imagine that for an extension with many functions in it, requiring @nonobjc on each one would get tedious very fast. Could it be required (or at least allowed in addition to per-method annotations) at the extension level?:
> 	
> 	@objc protocol P {}
> 	
> 	@nonobjc extension P {
> 		func foo() { }
> 		func bar() { }
> 		func baz() { }
> 		func blah() { }		
> 		// etc...
> 	}
> 
> I don’t know if this would have specific implementation ramifications over only doing this on each method, if extensions cannot already be modified with attributes. I can’t think of a case where I’ve seen annotations added to protocol extensions, or any other extensions for that matter.

We have some declaration modifiers (e.g., access-control modifiers) and attributes (e.g., availability) that distribute in this manner from the extension to its members. My only hesitation here is that @objc itself doesn’t distribute in this way, and I’d rather they not be inconsistent.

	- Doug

> 
>> On Jan 4, 2016, at 11:32 PM, Douglas Gregor via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> Hi all,
>> 
>> We currently have a bit of a surprise when one extends an @objc protocol:
>> 
>> @objc protocol P { }
>> 
>> extension P {
>>   func bar() { }
>> }
>> 
>> class C : NSObject { }
>> 
>> let c = C()
>> print(c.respondsToSelector("bar")) // prints "false"
>> 
>> because the members of the extension are not exposed to the Objective-C runtime. 
>> 
>> There is no direct way to implement Objective-C entry points for protocol extensions. One would effectively have to install a category on every Objective-C root class [*] with the default implementation or somehow intercept all of the operations that might involve that selector. 
>> 
>> Alternately, and more simply, we could require @nonobjc on members of @objc protocol extensions, as an explicit indicator that the member is not exposed to Objective-C. It’ll eliminate surprise and, should we ever find both the mechanism and motivation to make default implementations of @objc protocol extension members work, we could easily remove the restriction at that time.
>> 
>> 	- Doug
>> 
>> [*] Assuming you can enumerate them, although NSObject and the hidden SwiftObject cover the 99%. Even so, that it’s correct either, because the root class itself might default such a method, and the category version would conflict with it, so...
>> 
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> 

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