id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
5490	Fusion fails to insulate user-supplied template parameters from unintentional MPL placeholder substitution (?)	Jeffrey Hellrung <jeffrey.hellrung@…>	Kohei Takahashi	"The code below fails on MSVC9 with Boost 1.43.0, but recent developers' list activity {{{[1]}}} suggests this is still broken in Boost 1.46.1.  And, to emphasize, I believe it *should* compile, but I could be wrong there!

{{{
#include <iostream>

#include <boost/fusion/algorithm/transformation/transform.hpp>
#include <boost/fusion/container/vector/vector10.hpp>
#include <boost/fusion/sequence/io/out.hpp>
#include <boost/mpl/quote.hpp>
#include <boost/mpl/placeholders.hpp>
#include <boost/type_traits/is_integral.hpp>

template< class >
struct foo
{
    typedef int result_type;
    template< class T >
    int operator()(const T&) const
    { return 0; }
};


int main()
{
    boost::fusion::vector1< int > v;
    typedef boost::is_integral< boost::mpl::_1 > mpl_type; // 1
    //typedef boost::mpl::quote1< boost::is_integral > mpl_type; // 2
    std::cout << boost::fusion::transform(v, foo< mpl_type >());

    return 0;
}
}}}

A workaround is to use ""// 2"" rather than ""// 1"".

I believe the proper fix is to insulate any user-supplied template parameters from unintential Boost.MPL placeholder substitution within the implementation of boost::fusion::transform and/or boost::fusion::transform_view, but I haven't looked into where exactly this should occur or how widespread this is.

The existence of a workaround together with the probable rarity of the situation likely makes this a low-priority bug, but it is a subtle problem once you run into it.

{{{[1]}}} http://thread.gmane.org/gmane.comp.lib.boost.devel/218162
"	Bugs	closed	Boost 1.68.0	fusion	Boost 1.47.0	Problem	fixed	fusion, mpl, placeholder, substitution