class A
{
    bool p;

    ref bool? f(ref bool? z)
    {
        if (p) { return ref z; }
        else return ref z;
    }
}

class A
{
    bool p;

    ref bool? f(ref bool? z)
    {
        return p ? ref z : ref z; // CS8150    By-value returns may only be used in methods that return by value    

    }
}