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