Hi, Some higher order algorithms in the MPL have a default for the lambda expression they accept. A good example is `boost::mpl::equal`: template <typename S1, typename S2, typename Pred = is_same<_, _> > struct equal; This works fine most of the time, but I was recently bitten by the following: template <typename VectorOfVectors, typename Vector> struct find_vector : find_if< VectorOfVectors, equal<Vector, _1> > { }; typedef find_vector< vector< vector<int, int>, vector<char, char> >, vector<char, char> >::type ThisWillBreak; What happens here is that the `equal<Vector, _1>` expression inside `find_vector` really is `equal<Vector, _1, is_same<_1, _2> >` because of the default value for the predicate to `equal`. When the lambda is evaluated, the placholders inside the inner `is_same<_1, _2>` expression are replaced too, which yields unexpected results. Using template <typename S1, typename S2, typename Pred = typename lambda<is_same<_, _> >::type> struct equal; or equivalently template <typename S1, typename S2, typename Pred = quote2<is_same> > struct equal; fixes the issue. Also note that all of the unit tests of the MPL still pass with these changes. Is the current behavior intended, or should I submit a patch? Regards, Louis Dionne