variable.c:1557
static void
cvar_override_check(id, a)
    ID id;
    VALUE a;
{
    VALUE base = original_module(a);

    a = RCLASS(a)->super;
    while (a) {
	if (RCLASS(a)->iv_tbl) {
	    if (st_lookup(RCLASS(a)->iv_tbl,id,0)) {
		rb_warning("class variable %s of %s is overridden by %s",
			   rb_id2name(id), rb_class2name(original_module(a)),
			   rb_class2name(base));
	    }
	}
	a = RCLASS(a)->super;
    }
}
