The required: is problematic, because projects using libxml++ may also depend on libxml-2.0 themselves. That means meson will resolve the dep and cache it before it sees this Requires. So xml2_dep.found() will be true on windows. This means...

This winsock_dep, bcrypt_dep, and xml2_sp variables which are inside the if not xml2_dep.found() won't be set. xml2_sp is only used if build-tests is true, but there's no way around needing the winsock_dep variable.