Unfortunately there is currently a bug with Xcode previews in beta 1 that prevents the prefix / compatibility rules from working as expected. The rules do however work for a regular compilation, so should work fine for e.g running in the simulator. It also appears that syntax highlighting does not yet have the new rules.
Until fixed, one potential workaround to get previews working is to add a comment on the same line as the prefix operator use, e.g:
/x // dummy comment
This is also a known issue.