ty::Closure(..) | ty::Generator(..) => Tuple(vec![]), // FIXME(pcwalton)
+ ty::Bound(..) => panic!("Bound"),
ty::UnnormalizedProjection(..) => panic!("UnnormalizedProjection"),
ty::GeneratorWitness(..) => panic!("GeneratorWitness"),
ty::Infer(..) => panic!("Infer"),