The case is United States v. Davis. At issue is a federal statute which, in the Court's words, "threatens long prison sentences for anyone who uses a firearm in connection with certain other federal crimes. But which other federal crimes?" That is where the debate over vagueness comes in. The law itself calls for enhanced sentencing in cases involving felonies "that by [their] nature, involv[e] a substantial risk that physical force against the person or property of another may be used in the course of committing the offense."
And what exactly does that mean? Opinions differ. And therein lies the problem. As Justice Gorsuch pointed out in his majority opinion, "even the government admits that this language, read in the way nearly everyone (including the government) has long understood it, provides no reliable way to determine which offenses qualify as crimes of violence." And "in our constitutional order," Gorsuch observed, "a vague law is no law at all" because it violates the core constitutional requirement that all federal statutes "give ordinary people fair warning about what the law demands of them."