eval.c:1701
static VALUE
cvar_cbase()
{
    NODE *cref = ruby_cref;

    while (cref && cref->nd_next && FL_TEST(cref->nd_clss, FL_SINGLETON)) {
	cref = cref->nd_next;
	if (!cref->nd_next) {
	    rb_warn("class variable access from toplevel singleton method");
	}
    }
    return cref->nd_clss;
}
