How does the Apache 2 license's patent grant interact with third-party patents?

Question

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?

Answer 1

The license says:

Subject to the terms and conditions of this License, each Contributor hereby grants to You a perpetual, worldwide, non-exclusive, no-charge, royalty-free, irrevocable (except as stated in this section) patent license to make, have made, use, offer to sell, sell, import, and otherwise transfer the Work, where such license applies only to those patent claims licensable by such Contributor that are necessarily infringed by their Contribution(s) alone or by combination of their Contribution(s) with the Work to which such Contribution(s) was submitted.

Bold is mine. The way I interpret that is, the Apache license only means person A grants patent licenses that:

• A can legally grant
• That are necessary to use A's contribution, either alone or as part of the whole work

So it will still be necessary for C to obtain a license from B. But the license assures C that she won't be sued by A by using the software.