Pattern based async disposal does not work on lifted nullable structs

[chsienki]

When implementing async disposal via pattern, you should be able to use a nullable version of the struct in the await using statement:

Repro:

using System;
using System.Threading;
using System.Threading.Tasks;

namespace System
{
    public interface IAsyncDisposable
    {
        ValueTask DisposeAsync();
    }
}

public class C 
{
    public static async Task M()
    {
        StructDisposer? a = null;
        await using (a) { } // DisposeAsync is not invoked
        
        StructDisposer? b = new StructDisposer();
        await using (b) { } // DisposeAsync is invoked
    }
}

public struct StructDisposer /*: IAsyncDisposable*/
{
    public async ValueTask DisposeAsync() => Console.WriteLine("DisposeAsync");
}

Expected Behaviour:

Compiles successfully.

Actual Behavior:

error CS8410: 'StructDisposer?': type used in an async using statement must be implicitly convertible to 'System.IAsyncDisposable' or implement a suitable 'DisposeAsync' method.

Workaround:

Explicitly implement the IAsyncDisposable interface, and the code will compile and run successfully.

[jcouv update:] this issue is reference in the design doc. Should update the design doc when closing this issue.

[gafter]

The specification does not support pattern-based disposal of nullable structs. In fact, there is no specification for async using as a feature at all. See dotnet/csharplang#43 and https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/async-streams.md