id,summary,reporter,owner,description,type,status,milestone,component,version,severity,resolution,keywords,cc
8608,Default lambda expression with mpl::equal,Louis Dionne <ldionne.2@…>,Aleksey Gurtovoy,"Summary of the original message posted on the mailing list:

------------------------------------------------------------------------------
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.
------------------------------------------------------------------------------

Original thread:
http://thread.gmane.org/gmane.comp.lib.boost.devel/241690
",Bugs,new,To Be Determined,mpl,Boost Development Trunk,Problem,,default lambda mpl equal,