Share completed Task between Task and AsyncTaskMethodBuilder (#27437)

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

@@ -18,9 +18,6 @@ namespace System.Runtime.CompilerServices
     /// </remarks>
     public struct AsyncTaskMethodBuilder
     {
-        /// <summary>A cached VoidTaskResult task used for builders that complete synchronously.</summary>
-        private static readonly Task<VoidTaskResult> s_cachedCompleted = AsyncTaskMethodBuilder<VoidTaskResult>.s_defaultResultTask;
-
         /// <summary>The lazily-initialized built task.</summary>
         private Task<VoidTaskResult>? m_task; // Debugger depends on the exact name of this field.
 
@@ -103,7 +100,7 @@ public void SetResult()
             // If there isn't one, store the supplied completed task.
             if (m_task is null)
             {
-                m_task = s_cachedCompleted;
+                m_task = Task.s_cachedCompleted;
             }
             else
             {

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

@@ -1434,8 +1434,12 @@ WaitHandle IAsyncResult.AsyncWaitHandle
         /// </remarks>
         public static TaskFactory Factory { get; } = new TaskFactory();
 
+        /// <summary>Singleton cached task that's been completed successfully.</summary>
+        /// <remarks>It's a <see cref="Task{VoidTaskResult}"/> so it can be shared with <see cref="AsyncTaskMethodBuilder"/>.</remarks>
+        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.