Fix for #11. Cross port of fix for #1742 from vibe.d

source/vibe/core/task.d

@@ -56,7 +56,7 @@ struct Task {
 		auto fiber = () @trusted { return Fiber.getThis(); } ();
 		if (!fiber) return Task.init;
 		auto tfiber = cast(TaskFiber)fiber;
-		assert(tfiber !is null, "Invalid or null fiber used to construct Task handle.");
+		if (!tfiber) return Task.init;
 		// FIXME: returning a non-.init handle for a finished task might break some layered logic
 		return () @trusted { return Task(tfiber, tfiber.m_taskCounter); } ();
 	}

tests/vibe.core.core.1742.d

@@ -0,0 +1,50 @@
+/+ dub.sdl:
+    name "tests"
+    description "Semaphore hang"
+    dependency "vibe-core" path="../"
++/
+module test;
+import std.stdio;
+import std.socket;
+import std.datetime;
+import std.functional;
+import core.time;
+import vibe.core.core;
+import vibe.core.log;
+import vibe.core.concurrency;
+import vibe.core.connectionpool;
+
+class Conn {}
+
+void main()
+{
+	auto g = new Generator!int({
+		auto t = runTask({});
+		t.join();
+		yield(2);
+	});
+	assert(!g.empty);
+	assert(g.front == 2);
+	g.popFront();
+	assert(g.empty);
+
+	runTask({
+		auto g2 = new Generator!int({
+			auto t = runTask({});
+			t.join();
+			yield(1);
+		});
+		assert(!g2.empty);
+		assert(g2.front == 1);
+		g2.popFront();
+		assert(g2.empty);
+		exitEventLoop();
+	});
+
+	setTimer(5.seconds, {
+		assert(false, "Test has hung.");
+	});
+
+	runApplication();
+}
+