#nullable enable

namespace NoItDoesNotGetCalled

{

    using System;

    using System.Runtime.CompilerServices;

    using System.Threading.Tasks;

​

    [AsyncMethodBuilder(typeof(CustomAwaitableAsyncMethodBuilder<>))]

    public readonly struct CustomAwaitable<T>

    {

        readonly ValueTask<T> _valueTask;

​

        public CustomAwaitable(ValueTask<T> valueTask)

        {

            _valueTask = valueTask;

        }

​

        public ValueTaskAwaiter<T> GetAwaiter() => _valueTask.GetAwaiter();

    }

​

    public struct CustomAwaitableAsyncMethodBuilder<T>

    {

        public CustomAwaitable<T> Task => default;

​

        public void AwaitOnCompleted<TAwaiter, TStateMachine>(

            ref TAwaiter awaiter,

            ref TStateMachine stateMachine)

            where TAwaiter : INotifyCompletion

            where TStateMachine : IAsyncStateMachine =>

            awaiter.OnCompleted(stateMachine.MoveNext);

​

        public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(

            ref TAwaiter awaiter,

            ref TStateMachine stateMachine)

            where TAwaiter : ICriticalNotifyCompletion

            where TStateMachine : IAsyncStateMachine =>

            awaiter.UnsafeOnCompleted(stateMachine.MoveNext);

​

        public static CustomAwaitableAsyncMethodBuilder<T> Create() => default;

​

        public void SetException(Exception exception) {}

​

        public void SetResult(T result) {}

​

        public void SetStateMachine(IAsyncStateMachine stateMachine)

        {

            Console.WriteLine("Yet this doesn't happen :(");

        }

​

        public void Start<TStateMachine>(ref TStateMachine stateMachine)

            where TStateMachine : IAsyncStateMachine

        {

            if (typeof(TStateMachine).IsValueType)

            {

                Console.WriteLine("See? Here's a state machine that's implemented as a struct!");

            }

            stateMachine.MoveNext();

        }

    }

​

    public class Program

    {

        public static async Task Main()

        {

            async CustomAwaitable<object?> GetValueAsync()

            {

                await Task.CompletedTask;

                return null;

            }

            await GetValueAsync();

        }

    }

}