Lifting the "Self or associated type" constraint on existentials

[Joe_Groff]

@Karl

As for compiler optimisations, there is no intrinsic reason an existential must be less optimisable than a generic parameter. For example, we have a (relatively new) ExistentialSpecializer optimisation pass which transforms functions of the form func f(_: P) to func f<T:P>(_: T) , at which point our other generic specialisation infrastructure can take over.

Yeah, passing an existential as an argument is isomorphic to passing a generic argument with its own type variable—anything that's possible with one representation is possible on the other. We've even discussed giving them the same underlying ABI.

@Karl

Protocol existential self-conformance is a massive issue, though. I'll come back to this when I have some time to elaborate, but basically, I think it's a flaw in the design of the type-system. The type of an existential should be some kind of type-existential (i.e. "any type which conforms to P") rather than the protocol type P itself.

In cases where you have a single existential value like this, it should be possible to automatically open the existential and pass along the value inside as an argument that's required to conform to the protocol, rather than requiring the existential itself to conform. Would that address your use case?