Why does assumeIsolated() now require T: Sendable?

[ellie20]

@ktoso

During review of region isolation some folks thought that maybe a -> sending T would help resolve the issue, but that's sadly not the case -- it'd open up the same kinds of holes.

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:

@hborla

If we pursue the "transferring result type" future direction of this proposal, we could add an overload of assumeIsolated that accepts a closure returning a (strawman syntax) transferring T result type where T is not required to conform to Sendable.