Straight from Form 4473, which is the federal background check form. This was revised in 2017, after the decision Chris is referring to above. Bold text from the form, not added.  Question 11e.  (Note this isn't really new, the link is from 9/2016).

Are you an unlawful user of, or addicted to, marijuana or any depressant, stimulant, narcotic drug, or any other controlled substance? Warning: The use or possession of marijuana remains unlawful under Federal law regardless of whether it has been legalized or decriminalized for medicinal or recreational purposes in the state where you reside.