id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
6989	BOOST_PP_SEQ_ELEM/BOOST_PP_IF behave differently on different compilers	vanyacpp@…	No-Maintainer	"MSVC accepts the following code, but gcc reject it with error message that it is impossible to get second element of sequence with size = 1. The problem is that gcc and clang expand both HANDLE_SIZE_EQ_1 and HANDLE_SIZE_EQ_2 before expanding BOOST_PP_IF.

As it turned out MSVC and GCC get different versions of BOOST_PP_SEQ_ELEM. When MSVC get version of BOOST_PP_SEQ_ELEM for GCC it also show warning.

See connect.microsoft.com/VisualStudio/feedback/details/743894/possibly-incorrect-macro-expansion-order for details.

// boost version 1.49
#include <boost/preprocessor/comparison/not_equal.hpp>
#include <boost/preprocessor/seq/cat.hpp>
#include <boost/preprocessor/seq/elem.hpp>
#include <boost/preprocessor/seq/size.hpp>

#define HANDLE_SIZE_EQ_1(seq)

#define HANDLE_SIZE_EQ_2(seq)                                                     \
    BOOST_PP_SEQ_ELEM(1, seq)

#define TEST_IF(seq)        \
    BOOST_PP_IF(BOOST_PP_NOT_EQUAL(2, BOOST_PP_SEQ_SIZE(seq)) \
    , HANDLE_SIZE_EQ_1(seq)                 \
    , HANDLE_SIZE_EQ_2(seq))

TEST_IF((a))
"	Bugs	closed	To Be Determined	preprocessor	Boost 1.49.0	Problem	fixed