[boost] [MPL] Bug with default lambda expressions?

21 May 2013

      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  >
    struct equal;

This works fine most of the time, but I was recently bitten by the following:

    template 
    struct find_vector
        : find_if<
            VectorOfVectors, equal
        >
    { };

    typedef find_vector<
        vector<
            vector,
            vector
        >,
        vector
    >::type ThisWillBreak;

What happens here is that the `equal` expression inside
`find_vector` really is `equal >` 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  >::type>
    struct equal;

or equivalently

    template  >
    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