I ran the test program from the api docs in 0.8.0-beta.4.
https://vibed.org/api/vibe.core.concurrency/async

auto val = async({
		logInfo("Starting to compute value in worker task.");
		sleep(500.msecs); // simulate some lengthy computation
		logInfo("Finished computing value in worker task.");
		return 32;
	});

	logInfo("Starting computation in main task");
	sleep(200.msecs); // simulate some lengthy computation
	logInfo("Finished computation in main task. Waiting for async value.");
	logInfo("Result: %s", val.getResult());

Here's the result.

Starting computation in main task
Starting to compute value in worker task.
Finished computation in main task. Waiting for async value.
CoreTaskFiber was terminated unexpectedly: Joining tasks in foreign threads is currently not supported.
Program exited with code 255

Any chance this could get fixed? I was seriously relying on this feature working; it was mentioned the introductory book as working, and the online docs don't indicate a difficultly, so I incorrectly thought async was working.