diff --git a/jvm/src/main/scala/async/VThreadSupport.scala b/jvm/src/main/scala/async/VThreadSupport.scala
index d31d3812..4190e99d 100644
--- a/jvm/src/main/scala/async/VThreadSupport.scala
+++ b/jvm/src/main/scala/async/VThreadSupport.scala
@@ -6,6 +6,7 @@ import java.lang.invoke.{MethodHandles, VarHandle}
 import java.util.concurrent.locks.ReentrantLock
 import scala.annotation.unchecked.uncheckedVariance
 import scala.concurrent.duration.FiniteDuration
+import scala.annotation.constructorOnly
 
 object VThreadScheduler extends Scheduler:
   private val VTFactory = Thread
@@ -21,7 +22,7 @@ object VThreadScheduler extends Scheduler:
     val sr = ScheduledRunnable(delay, body)
     () => sr.cancel()
 
-  private class ScheduledRunnable(val delay: FiniteDuration, val body: Runnable^) extends Cancellable {
+  private class ScheduledRunnable(delay: FiniteDuration, body: Runnable^) extends Cancellable { 
     @volatile var interruptGuard = true // to avoid interrupting the body
 
     val th = VTFactory.newThread: () =>
diff --git a/shared/src/main/scala/async/Cancellable.scala b/shared/src/main/scala/async/Cancellable.scala
index 15dc07b4..ab9d1cce 100644
--- a/shared/src/main/scala/async/Cancellable.scala
+++ b/shared/src/main/scala/async/Cancellable.scala
@@ -1,8 +1,12 @@
 package gears.async
 
+import language.experimental.captureChecking
+
+import java.util.concurrent.atomic.AtomicLong
+
 /** A trait for cancellable entities that can be grouped. */
 trait Cancellable:
-
+  val id = Cancellable.Id()
   private var group: CompletionGroup = CompletionGroup.Unlinked
 
   /** Issue a cancel request */
@@ -28,6 +32,11 @@ trait Cancellable:
 end Cancellable
 
 object Cancellable:
+  opaque type Id = Long
+  private object Id:
+    private val gen = AtomicLong(0)
+    def apply(): Id = gen.incrementAndGet()
+
   /** A special [[Cancellable]] object that just tracks whether its linked group was cancelled. */
   trait Tracking extends Cancellable:
     def isCancelled: Boolean
diff --git a/shared/src/main/scala/async/CompletionGroup.scala b/shared/src/main/scala/async/CompletionGroup.scala
index 0a10b877..7f5700c9 100644
--- a/shared/src/main/scala/async/CompletionGroup.scala
+++ b/shared/src/main/scala/async/CompletionGroup.scala
@@ -1,14 +1,17 @@
 package gears.async
+import language.experimental.captureChecking
+
 import scala.collection.mutable
 import scala.util.Success
 
 import Future.Promise
+import scala.annotation.unchecked.uncheckedCaptures
 
 /** A group of cancellable objects that are completed together. Cancelling the group means cancelling all its
   * uncompleted members.
   */
 class CompletionGroup extends Cancellable.Tracking:
-  private val members: mutable.Set[Cancellable] = mutable.Set()
+  private val members: mutable.Set[(Cancellable^) @uncheckedCaptures] = mutable.Set[(Cancellable^) @uncheckedCaptures]()
   private var canceled: Boolean = false
   private var cancelWait: Option[Promise[Unit]] = None
 
@@ -29,14 +32,14 @@ class CompletionGroup extends Cancellable.Tracking:
     unlink()
 
   /** Add given member to the members set. If the group has already been cancelled, cancels that member immediately. */
-  def add(member: Cancellable): Unit =
+  def add(member: Cancellable^): Unit =
     val alreadyCancelled = synchronized:
       members += member // Add this member no matter what since we'll wait for it still
       canceled
     if alreadyCancelled then member.cancel()
 
   /** Remove given member from the members set if it is an element */
-  def drop(member: Cancellable): Unit = synchronized:
+  def drop(member: Cancellable^): Unit = synchronized:
     members -= member
     if members.isEmpty && cancelWait.isDefined then cancelWait.get.complete(Success(()))
 
@@ -50,8 +53,8 @@ object CompletionGroup:
   object Unlinked extends CompletionGroup:
     override def cancel(): Unit = ()
     override def waitCompletion()(using Async): Unit = ()
-    override def add(member: Cancellable): Unit = ()
-    override def drop(member: Cancellable): Unit = ()
+    override def add(member: Cancellable^): Unit = ()
+    override def drop(member: Cancellable^): Unit = ()
   end Unlinked
 
 end CompletionGroup