[swift-evolution] Passing class protocols to functions accepting AnyObject

[Joe Groff]

> On Dec 4, 2016, at 6:46 PM, Charles Srstka via swift-evolution <swift-evolution at swift.org> wrote:
> 
> The following currently does not work:
> 
> protocol P: class {}
> class C: P {}
> 
> func foo<T>(t: T) where T: AnyObject {
> 	print("foo")
> }
> 
> let p: P = C()
> 
> foo(t: p) // error: cannot invoke 'foo' with an argument list of type '(t: P)'
> 
> It seems to me that this ought to have been allowed, since P is declared as being a reference type and thus should have been able to satisfy the function’s requirements.
> 
> Is this worthy of writing a language proposal, or would this be considered a bug that should be sent through the radar system instead?

It's a limitation of the current implementation. `AnyObject` is taken as meaning that a conforming type has a representation that consists of a single refcounted pointer. Protocol existentials do not have a single-refcounted representation since the witness table for the conformances must also be carried around, therefore the protocol type does not conform to AnyObject's representation requirement.

-Joe