How to override async void method builder?

[timcassell]

I copied the default System.Runtime.CompilerServices.AsyncVoidMethodBuilder and changed the public methods to call my own system, and I tried to apply it to a method for testing, but my builder never gets used.

[System.Runtime.CompilerServices.AsyncMethodBuilder(typeof(PromiseVoidMethodBuilder))]
private static async void TestAsyncVoidOverride(Promise promise, System.Exception exception)
{
    await promise;
    throw exception;
}

PromiseVoidMethodBuilder implementation

namespace Proto.Promises.Async.CompilerServices
{
    public struct PromiseVoidMethodBuilder
    {
        private PromiseMethodBuilder _target;

        private PromiseVoidMethodBuilder(PromiseMethodBuilder target)
        {
            _target = target;
        }

        public static PromiseVoidMethodBuilder Create()
        {
            return new PromiseVoidMethodBuilder(PromiseMethodBuilder.Create());
        }

        public void Start<TStateMachine>(ref TStateMachine stateMachine) where TStateMachine : IAsyncStateMachine
        {
            _target.Start(ref stateMachine);
            _target.Task.Forget();
        }

        public void SetStateMachine(IAsyncStateMachine stateMachine)
        {
            _target.SetStateMachine(stateMachine);
        }

        public void AwaitOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine)
            where TAwaiter : INotifyCompletion
            where TStateMachine : IAsyncStateMachine
        {
            _target.AwaitOnCompleted(ref awaiter, ref stateMachine);
        }

        public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine)
            where TAwaiter : ICriticalNotifyCompletion
            where TStateMachine : IAsyncStateMachine
        {
            _target.AwaitUnsafeOnCompleted(ref awaiter, ref stateMachine);
        }

        public void SetResult()
        {
            _target.SetResult();
        }

        public void SetException(Exception exception)
        {
            _target.SetException(exception);
        }
    }
}

Is overriding async void method builder supported, or am I doing something wrong?

@stephentoub

P.S. I am able to override async ValueTask without issue. Using net6.0 and C# 10.

[stephentoub]

As far as I remember, this is not something we designed to be able to override. @jcouv would know for sure, but I doubt it's supported.

[jcouv]

This doesn't surprise me, but I couldn't find any explicit notes. Maybe @cston would remember.