[Boost-users] Bug in boost::phoenix ?

Subject: [Boost-users] Bug in boost::phoenix ?
From: Nicolas Savoire (nicos_at_[hidden])
Date: 2011-02-24 10:05:21

I am using boost::phoenix lambda functions and I am observing some weird behaviours 
when compiling with optimizations (-O3).

The first issue is that compiling the following piece of code 

Compiling:
  const float x = 1.0f;
  const float y = 1.0f;
  std::cout << (arg1+arg2)(x,y);

with g++ -Wall gives the warning: 
boost_1_46_0/boost/spirit/home/phoenix/operator/arithmetic.hpp:74:5: warning: returning reference to temporary

And I suspect a bug in the project I am working on is a consequence of this.
It is as if a boost::for_each( rng, lambda_fun ) was optimized out.
I have opened a ticket on this issue:
https://svn.boost.org/trac/boost/ticket/5190
I suspect the issue comes from the type deduction inside phoenix.
I first discovered the issue with boost 1.45.0, but it still here in 1.46.0.

The other issue I am facing is similar. Consider the following code:

namespace bp = boost::phoenix;
using boost::phoenix::arg_names::arg1;

int i=0;
int a[] = {0};

template<class UnaryFunction>
inline void foo( UnaryFunction fun )
{
  fun(i);
}

int main()
{
  foo( bp::ref(a)[arg1]++ );
  std::cout << a[0] << " ";  
  
  (bp::ref(a)[arg1]++)(i);
  std::cout << a[0] << "\n";  
}

Compiled without optimizations, it corectly prints: 1 2
Whereas compiled with g++-4.5.1 -O3 with boost 1.45.0, it prints: 0 1,
 as if the first lambda invocation was removed.

The issue disappears if I use g++-4.4 or boost 1.46.0. I tracked the changes 
between boost 1.45.0 and 1.46.0, and it seems related to 
https://svn.boost.org/trac/boost/changeset/66411
applying the part of the changeset concerning:
fusion/support/detail/is_mpl_sequence.hpp
fusion/support/is_sequence.hpp
fusion/support/sequence_base.hpp
to boost 1.45.0 fixes the issue, but I can't see how this is related to the
bug I observed.
Is it a compiler bug, a phoenix bug or a mistake in my code ?

Thanks,
nicolas