Anthony Sottile thank you for reporting this. At the moment we have ~80% Facter 3.x facts implemented. is_virtual fact on Linux is planned in FACT-2245.
yeah I was just very surprised since this is listed in many of the puppet docs and it trashed my machine when I upgraded and suddenly the wrong blocks were running (undefined facts are "falsey")
Ah Anthony Sottile, this issue occurs because you're using puppet as a gem, and it relaxed the facter dependency to allow facter-ng 4. If you override the gem constraint then you'll get facter 2.5.7 again.