The following two lines result in a build failure when using c++ modules with clang:
include/boost/di.hpp:2104:42: error: token is not a valid binary operator in a preprocessor subexpression
include/boost/di.hpp:2127:42: error: token is not a valid binary operator in a preprocessor subexpression
Not sure what compiler the 3'9 syntax works with, but clang-scan-deps-19 chokes on it.