CS8344 error even with C# 13 (ref in iterators/async)

[ufcpp]

Version Used:

VS 17.12.0 Preview 1.0
C# Tools 4.12.0-1.24379.11+cf82d399c36008e7936d545cde24141f8d3790fa

Steps to Reproduce:

static async Task M1()
{
    // CS8344.
    foreach (var r in new E1()) ;

    await Task.Yield();

    // No error with C# 13.
    E1 e1 = new();
    while (e1.MoveNext()) { var x = e1.Current; }

    await Task.Yield();

    // No error with C# 13.
    foreach (ref var x in new E2()) ;

    await Task.Yield();

    // No error with C# 13.
    E2 e2 = new();
    while (e2.MoveNext()) { ref var x = ref e2.Current; }
}

static IEnumerable<object?> M2()
{
    // CS8344.
    foreach (var r in new E1()) ;

    yield return null;

    // No error.
    E1 e1 = new();
    while (e1.MoveNext()) { var x = e1.Current; }

    yield return null;

    // No error with C# 13.
    foreach (ref var x in new E2()) ;

    yield return null;

    // No error with C# 13.
    E2 e2 = new();
    while (e2.MoveNext()) { ref var x = ref e2.Current; }
}

ref struct E1
{
    public E1 GetEnumerator() => this;
    public bool MoveNext() => false;
    public int Current => 0;
}

struct E2
{
    public E2 GetEnumerator() => this;
    public bool MoveNext() => false;
    public ref int Current => ref System.Runtime.CompilerServices.Unsafe.NullRef<int>();
}

Expected Behavior:

No error with the ref/unsafe in iterators/async feature.

Actual Behavior:

CS8344 error on the foreach.

I am not sure if this is a bug as it is not on the proposal document, but I would consider this to be a specification omission.

[jjonescz]

Thanks for reporting this. The feature did not change anything about the CS8344 error which disallows ref struct enumerators inside async/iterator methods (although the enumerable itself can be a ref struct). I think there is no reason for keeping this error; we could simply remove it in C# 13 and let the ref-in-async feature disallow unsafe usages (i.e., it would still disallow the foreach on ref struct enumerator if there are any awaits or yield returns in its body).

It doesn't look like the CS8344 error is part of the C# spec, so no spec changes are needed.