message: use modern concept definition
diff: @@ -224,13 +224,13 @@ CPP_PP_IGNORE_CXX2A_COMPAT_BEGIN
  #ifdef CPP_DOXYGEN_INVOKED
  #define CPP_PP_DECL_DEF_IMPL(TPARAM, NAME, ARGS, ...) \
  CPP_PP_CAT(CPP_PP_DEF_, TPARAM) \
  - concept bool NAME = CPP_PP_DEF_IMPL(__VA_ARGS__,)(__VA_ARGS__) \
  + META_CONCEPT NAME = CPP_PP_DEF_IMPL(__VA_ARGS__,)(__VA_ARGS__) \
  /**/
  #else
  #define CPP_PP_DECL_DEF_IMPL(TPARAM, NAME, ARGS, ...) \
  inline namespace _eager_ { \
  CPP_PP_CAT(CPP_PP_DEF_, TPARAM) \
  - concept bool NAME = CPP_PP_DEF_IMPL(__VA_ARGS__,)(__VA_ARGS__); \
  + META_CONCEPT NAME = CPP_PP_DEF_IMPL(__VA_ARGS__,)(__VA_ARGS__); \
  } \
  struct CPP_PP_CAT(NAME, Concept) { \
  CPP_PP_CAT(CPP_PP_DEF_, TPARAM) \
  