[notpushkin]

> bar calls foo. Isn't bar a derivative work of foo?

I think it isn’t. Function signatures are fair use (under certain conditions, as established in Google v. Oracle), so just calling a function isn’t enough to declare your code a derivative work.

The combined work however is of course a derivative work of foo and is subject to its license terms, so if you want to be able to distribute it, you have to allow abide by them (i.e. release source code for both foo and bar if foo is AGPL, even though bar can be Apache or MIT or something; and you can’t use a more restrictive license that AGPL for bar).

If you want you can reimplement foo yourself though – then you can distribute bar together with your implementation under any license you like.

(IANAL)