Hang when awaiting call to actor

[marco.masser]

There’s progress on this!

[Pitch] Clarify the execution of non-actor-isolated `async` functions

SE-0296 (async/await) never specified where non-actor-isolated async functions run; the proposal was already quite large without specifying those details. SE-0306 (actors) necessarily gave some rules about how actor-isolated async functions run, but it never quite specified the interaction with non-actor-isolated async functions. There's a pitch for custom executors for actors, but that's also only about actor-isolated functions. SE-0302 (Sendable) implies some rules about how these functions…