GlobalPtr.opAssign

{
    GlobalPtr!long x = new long(1);

    x = null;
    assert(x == null);
}

{
    GlobalPtr!(shared long) x = new shared(long)(1);

    x = null;
    assert(x == null);
}

{
    shared GlobalPtr!(long) x = new shared(long)(1);

    x = null;
    assert(x == null);
}