#nullable enable

public class Program
{
    static void M(bool flag, string? x)
    {
        if (flag ? x?.Length == 2 : x?.Length == 3)
        {
            x.ToString(); // warns today, but perhaps shouldn't warn
        }
    }
}

SharpLab

Note that if flag is changed to the constant true, the warning is not given.

The suggestion here is that if either conditional expression arm has a split state, we join the StateWhenTrue on each arm, and join the StateWhenFalse on each arm, and use the resulting split state as the resulting state of the conditional expression.