await foreach utilizing struct enumerator never ends

[AlekseyTs]

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

struct S1
{
    public S2 GetAsyncEnumerator(CancellationToken token = default)
    {
        return new S2();
    }
}

struct S2
{
    bool stop;

    public ValueTask DisposeAsync()
    { 
        System.Console.Write("D");
        return ValueTask.CompletedTask;
    }

    public int Current => 123;
    public async ValueTask<bool> MoveNextAsync()
    {
        if (!stop)
        {
            stop = true;
            return true;
        }

        return false;
    }
}


class C
{
    static async Task Main()
    {
        await Test(new S1());
    }

    static async Task Test(S1 s1)
    {
        await foreach (var i in s1)
        {
            System.Console.Write(i);
        }
    }
}

Expected output: "123D"

Observed: an infinite loop

If S2 is changed to a class, the code behaves as expected. It is quite possible that generated code makes unexpected copies of the struct.

A generic case also exhibits the same problem:

 #nullable disable

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

interface ICustomEnumerator
{
    public int Current {get;}

    public ValueTask<bool> MoveNextAsync();
}

interface IGetEnumerator<TEnumerator> where TEnumerator : ICustomEnumerator
{
    TEnumerator GetAsyncEnumerator(CancellationToken token = default);
}

struct S1 : IGetEnumerator<S2>
{
    public S2 GetAsyncEnumerator(CancellationToken token = default)
    {
        return new S2();
    }
}

struct S2 : ICustomEnumerator, IAsyncDisposable
{
    bool stop;

    public ValueTask DisposeAsync()
    { 
        System.Console.Write("D");
        return ValueTask.CompletedTask;
    }

    public int Current => 123;
    public async ValueTask<bool> MoveNextAsync()
    {
        if (!stop)
        {
            stop = true;
            return true;
        }

        return false;
    }
}


class C
{
    static async Task Main()
    {
        await Test<S1, S2>();
    }

    static async Task Test<TEnumerable, TEnumerator>()
        where TEnumerable : IGetEnumerator<TEnumerator>
        where TEnumerator : ICustomEnumerator, IAsyncDisposable
    {
        await foreach (var i in default(TEnumerable))
        {
            System.Console.Write(i);
        }
    }
}

[AlekseyTs]

CC @jcouv