Awaiter GetResult returning by reference

[KalleOlaviNiemitalo]

I was playing with this code in SharpLab:

using System;
public class C {
    public async void M() {
        N(ref await new RT());
    }
    private void N(ref int r) {}
}

public class RT {
    public Awaiter GetAwaiter() {
        return new Awaiter();
    }
    public struct Awaiter : System.Runtime.CompilerServices.INotifyCompletion {
        private static int k;
        public bool IsCompleted => true;
        public void OnCompleted(Action action) {
            action();
        }
        public ref int GetResult() {
            return ref k;
        }
    }
}

The ref await syntax causes an error:

error CS1510: A ref or out value must be an assignable variable

That seems to be covered by the existing wording:

12.9.8.3 Classification of await expressions

The expression await t is classified the same way as the expression (t).GetAwaiter().GetResult(). Thus, if the return type of GetResult is void, the await_expression is classified as nothing. If it has a non-void return type T, the await_expression is classified as a value of type T.

GetResult here returns int by reference, but the await_expression is classified as a value. So the compiler works as specified. Still, it might be good to add a note about this case.