[swift-evolution] Should we rename "class" when referring to protocol conformance?

[Andrew Trick]

> On May 7, 2016, at 6:43 PM, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
> 
> If I read Andrew’s post correctly it sounds like it may also be of use to the optimizer in some cases.


I’ll just requote Dave’s example, which made perfect sense to me (so I’m not sure why there’s an argument):

> To me that means, if the behavior of “f” only depends on
> data reachable through this array, and f makes no mutations, then in
> this code, the two calls to f() are guaranteed have the same effect.
> 
>      func g<T>(a: [T]) {
>        var vc = MyViewController(a)
>        vc.f() // #1
>        h()
>        vc.f() // #2
>     }
> 
> But clearly, the only way that can be the case is if T is actually
> immutable (and contains no references to mutable data), because
> otherwise anybody can write:
> 
>    class X { ... }
>    let global: [X] = [ X() ]
>    func h() { global[0].mutatingMethod() }
>    g(global)
> 
> Conclusion: your definition of PureValue, as written, implies conforming
> reference types must be immutable.  I'm not saying that's necessarily
> what you meant, but if it isn't, you need to try to define it again.

Yes, of course. If a PureValue contains a reference it must be immutable or only mutated when uniquely referenced.

There are other ways to communicate what the optimizer needs. I think the more interesting question is how users should express the value semantics of their types.

-Andy
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