SE-0309: Unlock existential types for all protocols

[anthonylatsis]

@Paul_Cantrell

Is there a snapshot build I can use to experiment with this?

Not yet (we'll post a link).

@Paul_Cantrell

First, is there any value in exposing non-covariant self as Never rather than making the method disappear altogether?

@Paul_Cantrell

I can also imagine that there might be situations where the compiler might be able to determine a better lower bound than Never from generic constraints etc. Does such a situation exist? (For example, the compiler might be able to deduce a less-restrictive bound for a private protocol, where all possible implementing types are known at compile time.)

Not sure there is practical value in going that far. Besides, exposing non-covariant Self as Never would definitely introduce a source compatibility impediment once we start considering path-dependent types. It is possible to deduce Self internally in the event of a single conformance, but this kind of inference may leave unintended type-erasure unnoticed until the next conformance (also, having a non-public protocol around for the sake of a single conformance is something you should most likely avoid in production code).

@Paul_Cantrell

However, if I understand correctly, the following code still won’t work even if this proposal is accepted:

let testShapes: [Shape] = […]
for shape in testShapes {
    assertDuplicateMatches(shape)
}

…because the compiler can’t determine a non-existential SpecificShapeType for the call to assertDuplicateMatches . Do I have that right?

Given that, I take it this implies the Shape existential does not actually conform to the Shape protocol? The proposal doesn’t say this explicitly, but it must be so. (Given that, what is the error message here? Seems like one that requires extra-special care.)

Given the above, I take it that this future direction in the proposal:

Make existential types "self-conforming" by automatically opening them when passed as generic arguments to functions. Generic instantiations could have them opened as opaque types.

…would make the code above work as written?

Correct. The error message should be the good ol' P does not conform to P (the self-conformance issue).