Compilation fails with using nlohmann::json (-DUSE_JSON)

When setting -DUSE_JSON by adding add_definitions(-DUSE_JSON) to the top-level CMakeLists.txt file, there is a compilation failure due to is_bool not being defined for nlohmann::json -- instead it is is_boolean. An example of is_bool being called is here.

I am unfamiliar with variant -- but what are the motivations? Is it related to MsgPack? If so -- what is wrong with the MsgPack serdes provided by nlohmann::json?