in Check.java we have

    /** Check that a type extends a list of types
     * @param pos Position to be used for error reporting.
     * @param a The subtype in the extends relationship.
     * @param b The supertype in the extends relationship.
     */
    public void checkExtends(int pos, Type a, List<Type> bs) {
        for (List<Type> l = bs; l.nonEmpty(); l = l.tail) {
            if (!a.isSubType(bs.head)) {
                boundError(pos, a, bs.head, "");
                return;
            }
        }
    }

I believe the uses of bs.head in the loop should be l.head.