The initializers for d1 and d2 should be equivalent. Currently, the initializer for d2 is interpreted as ref (int () => x) and is reported as an error.

delegate ref int D();

class Program
{
    static void Main()
    {
        D d1 = (ref int () => x);
        D d2 = ref int () => x;
    }
}

See LDM notes.