/// Begins an asynchronous coroutine, transferring control to `body` until it /// either suspends itself for the first time with `suspendAsync` or completes, /// at which point `beginAsync` returns. If the async process completes by /// throwing an error before suspending itself, `beginAsync` rethrows the error. func beginAsync(_ body: () async throws -> Void) rethrows -> Void

↑の戻り値ってなんで T じゃないの？