ty::Closure(did, substs) => ty::Closure(did, substs.try_fold_with(folder)?),
ty::Alias(kind, data) => ty::Alias(kind, data.try_fold_with(folder)?),
ty::Closure(did, substs) => ty::Closure(did, substs.try_fold_with(folder)?),
ty::Alias(kind, data) => ty::Alias(kind, data.try_fold_with(folder)?),