Generalized Async Return Types in C# 7.0 sample code.

CustomValueTask.cs

    [AsyncMethodBuilder(typeof(AsyncCustomValueTaskMethodBuilder<>))]
    [StructLayout(LayoutKind.Auto)]
    public struct CustomValueTask<T>: IEquatable<CustomValueTask<T>>
    {
        internal readonly T _result;
        internal readonly Task<T> _task;
        public CustomValueTask(Task<T> task)
        {
            _task = task;
            _result = default(T);
        }
        public CustomValueTask(T result)
        {
            _result = result;
            _task = (Task<T>) null;
        }

        public Task<T> AsTask()
        {
            return _task ?? Task.FromResult<T>(_result);
        }

        public bool IsCompleted
        {
            get
            {
                if (this._task != null)
                    return this._task.IsCompleted;
                return true;
            }
        }
        public MyAwaiter<T> GetAwaiter()
        {
            return new MyAwaiter<T>(this);
        }

        public bool Equals(CustomValueTask<T> other)
        {
            if (this._task == null && other._task == null)
                return EqualityComparer<T>.Default.Equals(this._result, other._result);
            return _task == other._task;
        }

        [EditorBrowsable(EditorBrowsableState.Never)] // intended only for compiler consumption
        public static AsyncValueTaskMethodBuilder<T> CreateAsyncMethodBuilder() => AsyncValueTaskMethodBuilder<T>.Create();
    }

MyAwaiter.cs

  public struct MyAwaiter<T> : INotifyCompletion
    {
        private readonly CustomValueTask<T> _customValueTask;

        public MyAwaiter(CustomValueTask<T> customValueTask)
        {
            _customValueTask = customValueTask;
        }

        public T GetResult()
        {
            if (_customValueTask._task != null)
                return _customValueTask._task.GetAwaiter().GetResult();
            return _customValueTask._result;
            
        }

        public bool IsCompleted => _customValueTask.IsCompleted;

        public void OnCompleted(Action continuation)
        {
            _customValueTask.AsTask().ConfigureAwait(true).GetAwaiter().OnCompleted(continuation);
        }
        
    }