Given a route like this:

book:
    path: /book
    defaults:
        page: 1

I would expect generate('book', {'page': $page}) to generate these routes:

1. /book when $page == 1
2. /book?page=2 when $page == 2

What actually happens is that when a parameter has a default value, but is not present in the route path, it just gets eaten by Symfony. Both of the above values for $page result in the path /page being generated.