The fact that we even have to have this discussion is a reason not to use it. I'd have to a hire a lawyer and disclose it to my investors to be cleared up. It would cost us money to validate because it does something different compared to any other similar project. My investors were very specific about software licenses - we aren't allowed to use GPLv4 or AGPL at all.

> I'd have to a hire a lawyer and disclose it to my investors to be cleared up

That seems extremely weird. Normal startups do nothing of the kind; do you have extremely weird investors?

> My investors were very specific about software licenses - we aren't allowed to use GPLv4 or AGPL at all.

Ah, there we go. First off, there's no such thing as GPLv4. Second of all, you do realise that avoiding even the AGPL puts you in a vanishingly rare crowd in these parts, right? Eg, MongoDB is AGPL; of all the reasons I've seen given for avoiding Mongo, the license is a new one on me.

If you've decided to only use GPLv2 or vanilla MIT licenses software that's totally fine, but it is unusual.

Sorry - meant GPLv3. All of the software we use is GPLv1/2, BSD, MIT, Apache etc..

If that's your concern, then you should comb through the source code of all your open source libraries and be certain that none of it violates any of Facebook's patents, including the MIT licensed stuff, which may very well.

You have it backwards.

If you have a patent, and use React, you can't sue Facebook if they use your patent.

> If you have a patent, and use React, you can't sue Facebook if they use your patent.

Citation needed. Let's say I have an awesome VR patent, and a React based website. I find out the Oculus violated my patent, what happens next?

Answer: I sue them. :) Nothing about the React license stops this; I will even still have a right to use React. What I won't have is a right to use any patents that Facebook might have on React. But Facebook, as far as is known, has no such patents.

From reacts/PATENTS

> The license granted hereunder will terminate, automatically and without notice, if you [...] initiate [...] any Patent Assertion

The key part is that if you don't have a license to use the patented technology (or more pertinently - that license is explicitly revoked), you can't legally use that technology, no matter what the BSD license might say: the patent grants supersedes the BSD license.

So, to respond directly to your answer - you lose the right to use React; Facebook can sue you for your use of React, since they own the patents to that technology.

So if you want to operate under the belief that Facebook has no patents to assert, then yes, you will consider yourself to be safe. The fact is that many lawyers, including the Apache Foundations lawyers, disagree with this assertion.

> The fact is that many lawyers, including the Apache Foundations lawyers, disagree with this assertion.

Incorrect. The ASF's objections had nothing to do with believing Facebook had any patents, and many ASF projects are looking to move to Preact, which is presumtively covered by the same patents React would be (if any).

But you can still sue Facebook. Nowhere does the PATENTS file require grantees to waive their right to sue. But if you do sue, Facebook gains the right to counter-sue you with any react related patents.

I.e. the exact situation you would be in if you sued Facebook and there was no PATENTS file.

Not quite - if there was no patents file, the BSD license would have a implied grant associated with it which would not be revoked by the lawsuit.

The "implied" license doesn't have a strong legal finding backing it up yet. Even so, most lawyers are comfortable with the idea that if a company tells other people to freely use their software, they can't then come back around and sue those same people.