Wrong compiler error regarding conditional conformances that involve Sendable about swift HOT 3 OPEN

cc @slavapestov

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Normally when you declare a conditional conformance you must explicitly declare the conformance to the inherited protocol, eg if you wanted your S to be Hashable you'd want to write something like this:

struct S<T> {}

extension S: Equatable where T: Equatable {}
extension S: Hashable where T: Hashable {}

This is because carrying over the bound to the inherited protocol (extension S: Equatable where T: Hashable) would be wrong.

However, an exception was made for Sendable to avoid breaking existing code; if some existing protocol is changed to inherit Sendable, and you conform to it conditionally, you get an implied Sendable conformance with the same conditional requirements.

You found the case where this rule doesn't work (which is why we don't want it in general, it's a backwards compatibility hack).

You can get around this by explicitly stating an unconditional Sendable conformance. The below type checks correctly for me:

protocol A: Sendable { }
protocol B: Sendable { }
struct S<T> { }
extension S: Sendable { }
extension S: A where T: A { }
extension S: B where T: B { }

from swift.

Normally when you declare a conditional conformance you must explicitly declare the conformance to the inherited protocol

🤦‍♂️ How did I forget that? I was too much focused on the "no breaking change" and slightly mislead by diagnostic messages. I should have mentioned this in my report.

However, an exception was made for Sendable to avoid breaking existing code.

Thank you for restating what Holly was saying in the forums.

You found the case where this rule doesn't work (which is why we don't want it in general, it's a backwards compatibility hack).

I very much welcome the intent, because it lets me do my job of OSS maintainer obsessed with backward compatibility :-) This also looks like a hack, but I was so relieved to see it.

You can get around this by explicitly stating an unconditional Sendable conformance.

Great 👍 This reduces the severity of the issue (for my own use case): if the issue still exists when Xcode 15.3 ships, I'll just mention the workaround in my documentation and guide users towards a solution.

Thank you very much @slavapestov!

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