[Version 13.0] Feature support for ref and unsafe in iterators and async

[RexJaeschke]

This is Rex's adaptation of the corresponding MS proposal.

[KalleOlaviNiemitalo]

OK, that means it is permissible to read a ref local variable in unreachable code.

using System.Collections.Generic;

public class C {
    public IEnumerator<int> M() {
        int i = 0;
        ref int r = ref i;

        yield return r; // OK
        // r is definitely unassigned, because of the yield return above.

        // yield return r; // error

        yield break;
        // r is again definitely assigned, because this point is unreachable.

        yield return r; // OK
    }
}

[KalleOlaviNiemitalo]

It is not clear to me that this disallows the following:

using System.Threading.Tasks;

public class C {
    public static async Task M(Task<int> t) {
        int i = 0;
        ref int r = ref i;
        K1(ref r, await t); // error
    }
    
    private static void K1(ref int r, int i) {}
}

One could claim that ref r is evaluated before the await expression and r is still definitely assigned at that point. So this depends on the "compiler-synthesized code" for await expressions but I'm not sure that is specified rigorously enough to say whether the above is allowed.