[boost] [Phoenix V3] for_each name conflict

Subject: [boost] [Phoenix V3] for_each name conflict
From: Mathieu - (ptr.jetable_at_[hidden])
Date: 2011-02-27 10:29:11

Hi everyone,

While reviewing phoenix V3 I encountered a bug while using mpl::for_each
with a phoenix lambda, because of a name conflict (lazy phoenix for_each
with mpl::for_each).
The following code reproduces the bug :

#include <boost/mpl/for_each.hpp>
#include <boost/mpl/vector_c.hpp>
#include <boost/phoenix/phoenix.hpp>
#include <iostream>

using boost::phoenix::arg_names::arg1;

int main(int argc, const char *argv[])
{
   typedef boost::mpl::vector_c<int,0,1,2,3> vec;

   boost::mpl::for_each<vec>(std::cout << arg1);

   return 0;
}

which fails to compile with the error :

In file included from main.cpp:1:
dev/phoenix3/boost/phoenix/stl/algorithm/iteration.hpp: In function
‘void boost::mpl::for_each <snip irrelevant long mpl / phoenix types>
main.cpp:25:   instantiated from here
dev/phoenix3/boost/phoenix/stl/algorithm/iteration.hpp:91: error:
‘boost::phoenix::for_each’ is not a function,
/usr/include/boost/mpl/for_each.hpp:109: error:   conflict with
‘template<class Sequence, class F> void boost::mpl::for_each(F,
Sequence*)’
/usr/include/boost/mpl/for_each.hpp:111: error:   in call to ‘for_each’

The mpl code does an unqualified call to for_each here :

void for_each(F f, Sequence* = 0)
{
   for_each<Sequence, identity<> >(f); // this is line 111
}

and phoenix provides :

   function<impl::for_each> const for_each = impl::for_each();

The (silly) change to mpl::for_each<Sequence, identity<> >(f); obviously
unbreak the build.

The problem seems to be that ADL kicks in and as f (in the mpl call of
for_each) is a phoenix type.

I don't have a patch (yet) as I don't really know the best strategy
here, nor I'm sure that my diagnostic is accurate (the ADL part that
is).

Mathieu-
:wq