The U.S. Supreme Court today decided Class v. United States, No. 16-424:

Does a guilty plea bar a criminal defendant from later appealing his conviction on the ground that the statute of conviction violates the Constitution? In our view, a guilty plea by itself does not bar that appeal.

Justice Breyer wrote the opinion of the Court.  Justice Alito dissented, joined by Justices Kennedy and Thomas, criticizing the majority for leaving a "muddle."