Implementing an task-like type with ref readonly in builder type

[OmarTawfik]

Part of test plan #19216

[jcouv]

I took a plain implementation of a task-like type builder and sprinkled ref readonly in a number of places.
In a couple of places (MyTask.GetAwaiter, MyTaskBuilder.Create, SetResult, SetException), I got errors.
But in the places with comments (see code below), I got no emit diagnostics, which I found surprising. I didn't try executing the code, since the methods have dummy implementations.

What do you think?

Tagging @cston @VSadov

        [Fact]
        public void AsyncTasklike_WithRefReadonly2()
        {
            var source = @"
using System;
using System.Runtime.CompilerServices;
class C
{
    static async MyTask<T> M<T>()
    {
        await M<T>();
        return default;
    }
}
[AsyncMethodBuilder(typeof(MyTaskBuilder<>))]
class MyTask<T>
{
    public Awaiter<T> GetAwaiter() => null;
}
class Awaiter<T> : INotifyCompletion
{
    public bool IsCompleted => true; 
    public T GetResult() => default;
    public void OnCompleted(Action completion) { } 
}
class MyTaskBuilder<T>
{
    public static MyTaskBuilder<T> Create() => null;

// ref readonly instead of ref
    public void Start<TStateMachine>(ref readonly TStateMachine stateMachine) where TStateMachine : IAsyncStateMachine { } 
    public void SetStateMachine(IAsyncStateMachine stateMachine) { }
    public void SetResult(T result) { }
    public void SetException(Exception exception) { }

// ref readonly return
    public ref readonly MyTask<T> Task => throw null; 

// ref readonly instead of ref
    public void AwaitOnCompleted<TAwaiter, TStateMachine>(ref readonly TAwaiter awaiter, ref readonly TStateMachine stateMachine) where TAwaiter : INotifyCompletion where TStateMachine : IAsyncStateMachine { } 

// ref readonly instead of ref
    public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(ref readonly TAwaiter awaiter, ref readonly TStateMachine stateMachine) where TAwaiter : ICriticalNotifyCompletion where TStateMachine : IAsyncStateMachine { } 
}
namespace System.Runtime.CompilerServices
{
    class AsyncMethodBuilderAttribute : System.Attribute { public AsyncMethodBuilderAttribute(System.Type t) { } }
}
";
            var compilation = CreateCompilation(source, options: TestOptions.ReleaseDll);
            compilation.VerifyEmitDiagnostics();
        }

We should also test ref struct and readonly struct as task-like.