Share CompletedTask between Task and AsyncTaskMethodBuilder

src/System.Private.CoreLib/shared/System/Runtime/CompilerServices/AsyncMethodBuilder.cs

@@ -191,13 +191,6 @@ private void NotifySynchronizationContextOfCompletion()
     /// </remarks>
     public struct AsyncTaskMethodBuilder
     {
-        /// <summary>A cached VoidTaskResult task used for builders that complete synchronously.</summary>
-#if PROJECTN
-        private static readonly Task<VoidTaskResult> s_cachedCompleted = AsyncTaskCache.CreateCacheableTask<VoidTaskResult>(default(VoidTaskResult));
-#else
-        private static readonly Task<VoidTaskResult> s_cachedCompleted = AsyncTaskMethodBuilder<VoidTaskResult>.s_defaultResultTask;
-#endif
-
         /// <summary>The generic builder object to which this non-generic instance delegates.</summary>
         private AsyncTaskMethodBuilder<VoidTaskResult> m_builder; // mutable struct: must not be readonly. Debugger depends on the exact name of this field.
 
@@ -270,7 +263,7 @@ public Task Task
         /// </summary>
         /// <exception cref="System.InvalidOperationException">The builder is not initialized.</exception>
         /// <exception cref="System.InvalidOperationException">The task has already completed.</exception>
-        public void SetResult() => m_builder.SetResult(s_cachedCompleted); // Using s_cachedCompleted is faster than using s_defaultResultTask.
+        public void SetResult() => m_builder.SetResult(Task.s_cachedCompleted); // Using s_cachedCompleted is faster than using s_defaultResultTask.
 
         /// <summary>
         /// Completes the <see cref="System.Threading.Tasks.Task"/> in the 

src/System.Private.CoreLib/shared/System/Threading/Tasks/Task.cs

@@ -1483,8 +1483,11 @@ internal TaskScheduler? ExecutingTaskScheduler
         /// </remarks>
         public static TaskFactory Factory { get; } = new TaskFactory();
 
+        // Is a Task{VoidTaskResult} so it can be shared with AsyncTaskMethodBuilder
+        internal static readonly Task<VoidTaskResult> s_cachedCompleted = new Task<VoidTaskResult>(false, default, (TaskCreationOptions)InternalTaskOptions.DoNotDispose, default);
+
         /// <summary>Gets a task that's already been completed successfully.</summary>
-        public static Task CompletedTask { get; } = new Task(false, (TaskCreationOptions)InternalTaskOptions.DoNotDispose, default);
+        public static Task CompletedTask => s_cachedCompleted;
 
         /// <summary>
         /// Provides an event that can be used to wait for completion.