The method has the same implementation as set_union():

  void set_union(const ArgumentMap &am) { _bits |= am._bits; }
  void set_intersect(const ArgumentMap &am) { _bits |= am._bits; }

Obviously, set_intersect() is wrong, should be:

  void set_intersect(const ArgumentMap &am) { _bits &= am._bits; }

It did not blow up anything, because it has no user.