NC 12 is not compatible with php 7.2 and a new feature support is only introduced by a new major version. So it will be supported by NC 13 which hasn’t been released yet. It originally was planned in 2017, there is already the beta3 version of it:
The final release date isn’t fixed yet and depends on the testing, it’s very likely in January.
There was a longer discussion on github when the first archlinux users switched to php 7.2:
https://github.com/nextcloud/server/issues/7384