Sendable loophole with `where Self: Sendable` extension

[sliemeobn]

Description

By creating an extension on a protocol with a where Self: Sendable constraint, you can use types as Sendable in the extension.

When you then conform a non-sendable type to another protocol that is witnessed by the where Self: Sendable extension mentioned above, things compile without errors.

Reproduction

protocol P {
    func doIt()
}

protocol F: AnyObject {
    var x: Int { get set }
}

class NonSendable: F {
    var x = 0
}

extension NonSendable: P {} // is this a bug?

extension F where Self: Sendable {
    func doIt() {
        Task { print(self.x) }
        x += 1
    }
}

func test() {
    let f: P = NonSendable()
    f.doIt()
}

Expected behavior

This should not compile unless the extension with the protocol conformance has the same where Self: Sendable constraint.

Environment

Apple Swift version 6.0.2 (swift-6.0.2-RELEASE)
Target: arm64-apple-macosx15.0

same behavior on
Apple Swift version 6.1-dev (LLVM 42f3e8ef873e24d, Swift c690fef)
Target: arm64-apple-macosx15.0

Additional information

No response

[slavapestov]

Reduced:

public protocol M {}

public protocol P {
    func doIt()
}

public class NonSendable: P {
}

@available(*, unavailable)
extension NonSendable: M {}

extension P where Self: M {
    public func doIt() {}
}

It appears that we don't walk into the witness substitution map when checking conformance availability, so we miss the NonSendable: M conformance being unavailable.

[tshortli]

Tracked by rdar://130308868.