Default lambda expression with mpl::equal

[Louis Dionne <ldionne.2@…>]

Summary of the original message posted on the mailing list:

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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.

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Original thread: ​http://thread.gmane.org/gmane.comp.lib.boost.devel/241690

[Louis Dionne <ldionne.2@…>]

Test case, code fix and documentation change.