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

[Vladimir.S]

Seems like ->StaticSelf can actually only means returns the class for which 
protocol conformance was applied first(in hierarchy). I.e. some `->BaseSelf`.
But in this case we need a method to get this 'base' class from protocol..

something like(just pseudo code!):

func makeWithZero<T: Makable >(x: Int) -> Makable(T).FirstConformedClass {
	return T.make(value: 0)
}

or

func makeWithZero<T: Makable >(x: Int) -> type<T.make(value:)> {
	return T.make(value: 0)
}

I don't know if all of this makes sense at all ;-)

On 13.05.2016 23:12, Joe Groff via swift-evolution wrote:
>
>> On May 13, 2016, at 9:06 AM, Matthew Johnson <matthew at anandabits.com> wrote:
>>
>>
>>> On May 13, 2016, at 10:55 AM, Joe Groff <jgroff at apple.com> wrote:
>>>
>>>
>>>> On May 13, 2016, at 8:18 AM, Matthew Johnson <matthew at anandabits.com> wrote:
>>>>
>>>> When I write a class Base with non-final methods that return instances of Base I can choose whether to state the return type as Self (covariant) or Base (invariant, under this proposal StaticSelf would also be an alternative way to state this).  If I choose to specify Base as the return type derived classes *may* override the method but are not required to.  Further, if they *do* override the method they are allowed to choose whether their implementation returns Base or Derived.
>>>
>>> `StaticSelf` requirements by themselves don't even save you from covariance. If Base conforms to a protocol (with Self == Base), Derived inherits that conformance and also conforms (with Self == Derived). If `StaticSelf` always refers to the conforming type, then it must also be bindable to Base and Derived, so a base class must still use a covariant-returning method to satisfy the `StaticSelf` requirement.
>>
>> We are specifying that `StaticSelf` refers to the type that explicitly declares conformance.  If a class inherits conformance it refers to the base class which explicitly declared the conformance it is inheriting.
>
> That makes `StaticSelf` tricky to use in generic code. This would be invalid:
>
> protocol Makable {
> 	static func make(value: Int) -> StaticSelf
> }
>
> func makeWithZero<T: Fooable>(x: Int) -> T {
> 	return T.make(value: 0) // ERROR: T.StaticSelf may be a supertype of T so isn't convertible to T
> }
>
> `StaticSelf` in this model is effectively an associated type of the protocol, with a `Self: StaticSelf` constraint (if that were supported).
>
> -Joe
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