Why does assumeIsolated() now require T: Sendable?

[John_McCall]

@ellie20

This may be a dumb question, but how? As far as I know, an instance of sending T will never be part of an actor isolation region (and, if T is Sendable, then any instance of T is also a sending T, because Sendable values are always disconnected). This was suggested as a future direction in the review thread, and as far as I know, no one suggested it would be unsound:

I believe that's still correct and that a sending result would be okay here. We do need to have some restrictions so that you can't merge the isolated region with the enclosing non-isolated region, though. For MainActor.assumeIsolated, this should be taken care of by the fact that the function is @Sendable; we need to check if those restrictions are in place for the general function.