From what I've read about the Apache 2 license's patent grant clause, it's intended to protect users of Apache-licensed software from software patents. How does that work if the contributor doesn't own the relevant patents, and thus can't grant the patent license required by the patent grant clause?
Suppose that person A obtains a license to use person B's software patent, and writes some code that uses it. Can person A then distribute that code under the Apache 2.0 license?
Suppose that person C then uses that code. Who's responsible for obtaining the patent license from person B?