Amending SE-0143: conditional conformances don't imply other conformances

[huon (Huon Wilson)]

@Paul_Cantrell

Even if the door is firmly shut on overlapping conformances, is there still something to be said for that approach? I find it less surprising — even expected, if I hadn’t read this proposal — that the compiler would infer Y: P where T: R as opposed to inferring A: R in the func foo<A>(x: Z<Y<A>>) example.

Yeah, I agree that that example is a bit subtle and a bit of an edge case. I actually realised a slightly more convincing one: SomeConditionalSequence<T>.Iterator will infer the appropriate bounds to make SomeConditionalSequence<T> a Sequence, to get access to Iterator.

@Paul_Cantrell

If the compiler didn’t make that foo inference, if it required func foo<A: P> instead, then could it safely assume Y: P where T: R (even without overlapping conformance)?

Or if not that, then could it make an inference more like “there exists some supertype U of R such that Y: P where T: U”?

I think it could for the specific case of a single conformance to a subprotocol of P, but as soon as you have more than one, an explicit conformance is needed. I don't think the supertype inference is useful: the only thing one can use with that in practice is T: R, as it's not known which parts of R and its supertypes are used in the conformance.