[swift-evolution] [Draft] Introducing StaticSelf, an Invariant Self

[Joe Groff]

> On May 12, 2016, at 5:49 PM, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
> 
> The invariant StaticSelf identifier will always refer to A, unlike Self, which is covarying and refers to
> the type of the actual instance. Since multiple inheritance for non-protocol types is disallowed,
> this establishes this invariant type identifier with no possibility for conflict.
> 
> Consider the following example, under the current system:
> 
> protocol StringCreatable 
> {
>     
> static func createWithString(s: String) -> Self
> 
> }
> 
> 
> extension NSURL: StringCreatable 
> {
>  
> // cannot conform because NSURL is non-final
> 
>  
> // error: method 'createWithString' in non-final class 'NSURL' must return `Self` to conform to protocol 'A'
> 
> }
> 
> Introducing a static, invariant version of Self permits the desired conformance:
> 
> protocol StringCreatable 
> {
>     
> static func createWithString(s: String) -> StaticSelf
> 
> }
> 
> 
> extension NSURL: StringCreatable 
> {
>  
> // can now conform conform because NSURL is fixed and matches the static
> 
>  
> // type of the conforming construct. Subclasses need not re-implement
> 
>  
> // NOTE: the return type can be declared as StaticSelf *or* as NSURL
> 
>  
> //       they are interchangeable
> 
>  
> static func createWithString(s: String) -> StaticSelf
>  { 
>      
> // ...
> 
>  }
> }
> 
> 

As I've noted before, I don't think this makes sense to encode in the protocol. `Self` is already effectively invariant within a protocol. If a protocol doesn't have the foresight to use StaticSelf, then you still have the same problems retroactively conforming class hierarchies to the protocol. Whether a conformance is inherited or not feels more natural as a property of a conformance, not something that can be legislated a priori by a protocol definition.

Something like StaticSelf might still be useful as shorthand within a class definition with a long-winded name, though `StaticSelf` feels kind of long as a shortcut to me.

-Joe