Don't automatically initialize Cancellable.group to Unlinked

Resolves infinite initialization loop. We need to check for its nullness when doing a re-link.

Author: natsukagami

shared/src/main/scala/async/Cancellable.scala

@@ -3,15 +3,15 @@ package gears.async
 /** A trait for cancellable entities that can be grouped. */
 trait Cancellable:
 
-  private var group: CompletionGroup = CompletionGroup.Unlinked
+  private var group: CompletionGroup = scala.compiletime.uninitialized
 
   /** Issue a cancel request */
   def cancel(): Unit
 
   /** Add this cancellable to the given group after removing it from the previous group in which it was.
     */
   def link(group: CompletionGroup): this.type = synchronized:
-    this.group.drop(this)
+    if this.group != null then this.group.drop(this)
     this.group = group
     this.group.add(this)
     this

shared/src/main/scala/async/CompletionGroup.scala

@@ -46,7 +46,7 @@ object CompletionGroup:
   /** A sentinel group of cancellables that are in fact not linked to any real group. `cancel`, `add`, and `drop` do
     * nothing when called on this group.
     */
-  val Unlinked = new CompletionGroup:
+  object Unlinked extends CompletionGroup:
     override def cancel(): Unit = ()
     override def waitCompletion()(using Async): Unit = ()
     override def add(member: Cancellable): Unit = ()