Bad diagnostic for ref struct using as awaiter

[huoyaoyuan]

Version Used: VS17.11.2, LangVersion preview

Steps to Reproduce:

    public ref struct RefAwaitable
    {
        public ref struct Awaiter : ICriticalNotifyCompletion, INotifyCompletion
        {
            public void UnsafeOnCompleted(Action continuation) => ThreadPool.UnsafeQueueUserWorkItem(_ => continuation(), null);
            public void OnCompleted(Action continuation) => ThreadPool.UnsafeQueueUserWorkItem(_ => continuation(), null);

            public bool IsCompleted => false;

            public int GetResult() => 114514;
        }

        public Awaiter GetAwaiter() => default;
    }

    internal class Program
    {
        static async Task Main(string[] args)
        {
            RefAwaitable r = default;
            Console.WriteLine(await r);
        }
    }

The code snippets errors for "RefAwaitable.Awaiter doesn't implement INotifyCompletion", though it can implement from C# 13. Since awaiter needs to be saved in state machine, it can't be ref struct.
Also, the ECMA C# 7.x spec doesn't call out that awaiter can't be ref struct. The spec may need to be updated together.