I know your tests are currently not being run on 2019. I gave it a shot and tried to compile anyways. I got a bunch of warnings and an error in one of the examples. Error was easy enough to fix (missing include) but the warnings are mostly about template magic in some system header. I don't get it exactly but seems to be originating from the utf8 conversion somehow.
..so VS2019 build would be awesome to have. Thanks for sharing this great lib.