Pure null check in binary operator doesn't affect 'else' branch

Version Used: e7bad77

Steps to Reproduce: SharpLab link

#nullable enable
public class C {
    public void M(string a, string b) {
        if (a is null && b is null)
        {
        }
        else
        {
            a.ToString(); // warning
            b.ToString(); // should warn, but doesn't
        }
        a.ToString(); // warning
        b.ToString(); // warning
    }
}

I don’t know why ‘a’ is considered maybe-null in the else branch here but not ‘b’. One key detail to note is that the declared types are non-nullable.

[RikkiGibson]

Check NullableWalker.LearnFromNullTest (I think) and NullableWalker.VisitBinaryOperatorChildren. I would debug and add a watch for this, step through, and see how it is that we make it into the else block in a state where b is not-null.

[RikkiGibson]

Weird, maybe there’s actually a different code path for &&. Given that I would break on ReportPossibleNullReceiverIfNeeded and check the call stack, or if even that fails break on DiagnosticBag.Add 😉