2008/6/18 Chris Fairles
I'm wrong. It's correctly erroring because: #elif 0 && () == 1 isn't a valid constant expression.
If BOOST_PP_ITERATION_DEPTH was not defined that would probably be: #elif 0 && BOOST_PP_ITERATION_DEPTH() == N Which I think still isn't valid. The basic structure of boost/mpl/aux_/advance_backward.hpp is something like: #if !defined(BOOST_PP_IS_ITERATING) // ... #elif BOOST_PP_ITERATION_DEPTH() == 1 // ... #elif BOOST_PP_ITERATION_DEPTH() == 2 // ... #endif If I understand you correctly, gcc 4.4 has started evaluating the '#elif' arguments even when a previous '#if' or '#elif' argument was true. I have no idea if gcc 4.4 is correct.... Your patch changes this to: #if !defined(BOOST_PP_IS_ITERATING) // ... #elif defined(BOOST_PP_ITERATION_DEPTH) # if BOOST_PP_ITERATION_DEPTH() == 1 // ... # endif #elif defined(BOOST_PP_ITERATION_DEPTH) # if BOOST_PP_ITERATION_DEPTH() == 2 // ... # endif #endif Which is wrong because the check 'BOOST_PP_ITERATION_DEPTH() == 2' can never be reached. A solution might be something like: #if !defined(BOOST_PP_IS_ITERATING) // ... #else # if BOOST_PP_ITERATION_DEPTH() == 1 // ... # elif BOOST_PP_ITERATION_DEPTH() == 2 // ... # endif #endif Anyway, I doubt the relevant developers will see this. The best way to get their attention would probably be to create a ticket: http://svn.boost.org/trac/boost/newticket?component=mpl Daniel