Sat, 27 Jan 2018 09:10:21 GMT

It seems that the reason is

BOOST_PP_TUPLE_ELEM(2, (El1))

doesn't compile with an error

error: macro "BOOST_PP_VARIADIC_ELEM_2" requires 4 arguments, but only 2 given

in the same time

BOOST_PP_TUPLE_ELEM(2, (El1,))

compiles

so, I have simplified the repro a bit:

#include <boost/preprocessor.hpp>
#define TUPLE_WITH_ONE_ELEMENT_WITHOUT_COMMA_AT_THE_END (a)
#define TUPLE_WITH_ONE_ELEMENT_WITH_COMMA_AT_THE_END (a,)
#define DEFINE_VARIABLE(tupleWithVariable)												\
    BOOST_PP_IF(0,                                                                      \
        BOOST_PP_TUPLE_ELEM(2, tupleWithVariable),                                      \
        BOOST_PP_TUPLE_ELEM(0, tupleWithVariable)                                       \
    )
int DEFINE_VARIABLE(TUPLE_WITH_ONE_ELEMENT_WITHOUT_COMMA_AT_THE_END); // doesn't compile
int DEFINE_VARIABLE(TUPLE_WITH_ONE_ELEMENT_WITH_COMMA_AT_THE_END);
int main() {
	return 0;
}

Boost C++ Libraries: Ticket #13372: Boost Preprocessing: compiler error: macro "BOOST_PP_VARIADIC_ELEM_2" requires 4 arguments, but only 2 given