Inconsistent dispose behavior between await using and await foreach

[AlekseyTs]

Execute the following code and examine its output (sharplab):

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

class C
{
    public static async Task Main()
    {
        Console.Write("Using dispose ");
        await using (new AsyncEnumerator())
        {
        }

        Console.Write("Foreach dispose ");
        await foreach (var i in new AsyncEnumerable())
        {
        }
    }
}

struct AsyncEnumerable : IAsyncEnumerable<int>
{
    public AsyncEnumerator GetAsyncEnumerator(CancellationToken token = default) => new AsyncEnumerator();
    IAsyncEnumerator<int> IAsyncEnumerable<int>.GetAsyncEnumerator(CancellationToken token) => new AsyncEnumerator();
}

struct AsyncEnumerator : IAsyncEnumerator<int>
{
    public int Current => 0;
    public async ValueTask<bool> MoveNextAsync()
    {
        await Task.Yield();
        return false;
    }
    public async ValueTask DisposeAsync()
    {
        Console.WriteLine("Pattern");
        await Task.Yield();
    }

    int IAsyncEnumerator<int>.Current => throw null;
    ValueTask<bool> IAsyncEnumerator<int>.MoveNextAsync() => throw null;
    async ValueTask IAsyncDisposable.DisposeAsync()
    {
        Console.WriteLine("Interface");
        await Task.Yield();
    }
}

Observed: Using gives priority to IAsyncDisposable implementation, foreach ignores IAsyncDisposable implementation.

Using dispose Interface
Foreach dispose Pattern

Expected: Both give priority to IAsyncDisposable implementation

Using dispose Interface
Foreach dispose Interface

[AlekseyTs]

@jcouv FYI

[jcouv]

This seems to be by-design and explicitly tested (DisposePatternPreferredOverIAsyncDisposable). See related issue #59955
The spec says we should bind a pattern-based DisposeAsync first, and if that doesn't work then try converting to interface, and if that doesn't work emit an empty finally: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-8.0/async-streams.md#semantics

[AlekseyTs]

I saw the tests, the behavior didn't look sound to me anyway.