Re: [Boost-bugs] [Boost C++ Libraries] #13372: Boost Preprocessing: compiler error: macro "BOOST_PP_VARIADIC_ELEM_2" requires 4 arguments, but only 2 given

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #13372: Boost Preprocessing: compiler error: macro "BOOST_PP_VARIADIC_ELEM_2" requires 4 arguments, but only 2 given
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2018-01-27 09:10:21

#13372: Boost Preprocessing: compiler error: macro "BOOST_PP_VARIADIC_ELEM_2"
requires 4 arguments, but only 2 given
------------------------------------+---------------------------
  Reporter:  Dmitrii B <d4qcdes@…>  |      Owner:  No-Maintainer
      Type:  Bugs                   |     Status:  new
 Milestone:  To Be Determined       |  Component:  preprocessor
   Version:  Boost 1.64.0           |   Severity:  Problem
Resolution:                         |   Keywords:
------------------------------------+---------------------------

Comment (by Dmitry B <d4qcdes@…>):

 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;
 }
 }}}

-- 
Ticket URL: <https://svn.boost.org/trac10/ticket/13372#comment:1>
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