boost/1.85.0 compile issues with msm::front::state_machine_def on llvm/clang++ 18.1.5 with std=c++20

[hoyhoy]

We have some code that uses

struct front_msm : msm::front::state_machine_def<front_msm, visitable_state> {...}
struct visitable_state {}
typedef boost::msm::back::recursive_get_transition_table<front_msm>::type recursive_stt;

typedef boost::msm::back::recursive_get_transition_table<Sm_rund_front>::type recursive_stt;
typedef boost::msm::back::generate_state_set<recursive_stt>::type all_states;

This is erroring out with boost 1.85.0 on the latest clang. boost 1.84.0 compiles successfully though.

clang++ 18.1.5 errors out with this on the compile.

/include/boost/msm/back/args.hpp:60:45: error: 'no_args' does not refer to a value [Semantic Issue]
   60 | BOOST_PP_REPEAT(BOOST_MSM_VISITOR_ARG_SIZE, MSM_VISITOR_ARGS, ~)
      |                                             ^
/include/boost/preprocessor/repetition/repeat.hpp:30:26: note: expanded from macro 'BOOST_PP_REPEAT'
   30 | # define BOOST_PP_REPEAT BOOST_PP_CAT(BOOST_PP_REPEAT_, BOOST_PP_AUTO_REC(BOOST_PP_REPEAT_P, 4))
      |                          ^
/include/boost/preprocessor/cat.hpp:22:32: note: expanded from macro 'BOOST_PP_CAT'
   22 | #    define BOOST_PP_CAT(a, b) BOOST_PP_CAT_I(a, b)
      |                                ^
/include/boost/preprocessor/cat.hpp:29:34: note: expanded from macro 'BOOST_PP_CAT_I'
   29 | #    define BOOST_PP_CAT_I(a, b) a ## b
      |                                  ^
<scratch space>:49:1: note: expanded from here
   49 | BOOST_PP_REPEAT_1
      | ^
/include/boost/msm/back/args.hpp:31:8: note: declared here
   31 | struct no_args {};
      |        ^
/include/boost/msm/back/args.hpp:60:45: error: use of undeclared identifier 'MSM_ARGS_PRINT' [Semantic Issue]
   60 | BOOST_PP_REPEAT(BOOST_MSM_VISITOR_ARG_SIZE, MSM_VISITOR_ARGS, ~)
      |                                             ^
/include/boost/preprocessor/repetition/repeat.hpp:30:26: note: expanded from macro 'BOOST_PP_REPEAT'
   30 | # define BOOST_PP_REPEAT BOOST_PP_CAT(BOOST_PP_REPEAT_, BOOST_PP_AUTO_REC(BOOST_PP_REPEAT_P, 4))
      |                          ^
/include/boost/preprocessor/cat.hpp:22:32: note: expanded from macro 'BOOST_PP_CAT'
   22 | #    define BOOST_PP_CAT(a, b) BOOST_PP_CAT_I(a, b)
      |                                ^
/include/boost/preprocessor/cat.hpp:29:34: note: expanded from macro 'BOOST_PP_CAT_I'
   29 | #    define BOOST_PP_CAT_I(a, b) a ## b
      |                                  ^
<scratch space>:49:1: note: expanded from here
   49 | BOOST_PP_REPEAT_1
      | ^

[hoyhoy]

msvc 17.9.4 had problems with this code on boost 1.84.0 as well, but that seemed to get fixed at 17.9.6.
https://developercommunity.visualstudio.com/t/cl-193933523-x64-boost1840-boost/10622335

[Lastique]

Please, report this to Boost.MSM.

[hoyhoy]

Seems like args.hpp is missing #include <boost/preprocessor/repetition/enum.hpp>

[hoyhoy]

Even with the include our boost::msm::back::state_machine is now going into an infinite recursion and crashing. Worked in boost/1.84.0