Merge pull request #13907 from stephentoub/valuetask_astask_caching

Use AsyncTaskCache in ValueTask.AsTask()

Author: stephentoub

src/mscorlib/shared/System/Threading/Tasks/ValueTask.cs

@@ -114,7 +114,7 @@ public Task<TResult> AsTask() =>
             // Return the task if we were constructed from one, otherwise manufacture one.  We don't
             // cache the generated task into _task as it would end up changing both equality comparison
             // and the hash code we generate in GetHashCode.
-            _task ?? Task.FromResult(_result);
+            _task ?? AsyncTaskMethodBuilder<TResult>.GetTaskForResult(_result);
 
         /// <summary>Gets whether the <see cref="ValueTask{TResult}"/> represents a completed operation.</summary>
         public bool IsCompleted => _task == null || _task.IsCompleted;

src/mscorlib/src/System/Runtime/CompilerServices/AsyncMethodBuilder.cs

@@ -719,7 +719,7 @@ internal void SetNotificationForWaitCompletion(bool enabled)
         /// <param name="result">The result for which we need a task.</param>
         /// <returns>The completed task containing the result.</returns>
         [MethodImpl(MethodImplOptions.AggressiveInlining)] // method looks long, but for a given TResult it results in a relatively small amount of asm
-        private Task<TResult> GetTaskForResult(TResult result)
+        internal static Task<TResult> GetTaskForResult(TResult result)
         {
             Contract.Ensures(
                 EqualityComparer<TResult>.Default.Equals(result, Contract.Result<Task<TResult>>().Result),