Protocol requirements co/contra variance

[tera]

In the following example:

class Base {}
class Derived: Base {}

class C {
    func foo(_ v: Base) {}
    func bar() -> Derived { Derived() }
}

I can call C's foo with instances of Derived:

let c = C()
c.foo(Derived())

and I can treat C's bar as returning instances of Base:

let v: Base = c.bar()

yet if I try to formally expressed these abilities in a form of protocol:

protocol P: AnyObject {
    func foo(_ v: Derived)
    func bar() -> Base
}

I can't conform C to this protocol:

extension C: P {} // 🛑 Type 'C' does not conform to protocol 'P'

apparently because protocol conformance requires exact type matching (unlike the co/contra variance flexibility allowed in subclass methods overrides).

The question is: shouldn't swift allow it?

[Jumhyn (Frederick Kellison-Linn)]

Old (but relevant) thread from a couple years back:

Protocol Witness Matching Mini-Manifesto

Hello Swift community, I have been working on a "mini-manifesto" whose aim is to set out basic design principles around protocol witness matching and get some agreement from the Swift community on it. One of the reasons why is because I am currently pitching a new form of witness matching and while we've had some discussion on this forum on different kinds of matching (like subtyping, etc), it would be great to have a small document which people can refer to during discussions and use it to cre…

[suyashsrijan]

This reminded me it would be good to revive it again (seems like my PR got closed bc I had to recreate the fork) and hopefully get it merged.

[cryo (Sune Foldager)]

Any news on this?

[suyashsrijan]

Yes I created a new thread here a while back - Protocol Witness Matching Roadmap