Soundness hole in isolated protocol conformances

[xwu (Xiaodi Wu)]

Are you on an ABI-stable platform with an older runtime? If so, this is a known—and unpluggable—hole:

@xwu

• While backdeployment on specific platforms is generally out of scope for Evolution, the steering group does acknowledge that the behavior of dynamic casting on older runtimes does present an issue affecting the user experience. Namely, on runtimes that pre-date isolated protocol conformances, casting a value of type C: @MainActor P to any P either must succeed where sometimes it should not, admitting data races, or it must fail where it should not, rendering the feature broadly unusable. The steering group agrees that the former behavior is nonetheless a net move in the right direction; it would fall upon tooling to provide opt-in diagnostics that can help users identify backdeployment issues.