Since the explosion in generative AI, there has been a rash of "decompilations" of video games and various other software (~~such as this example from earlier today~~ disputed -- see below) that have been published to Github and advertised as "Open Source." That claim is a lie.
The phrase "Open Source" has a specific meaning (and similarly for "Free Software," by the way^1^), and it isn't merely that the source code is there for you to look at. It means that the copyright holder is explicitly giving you permission to read that source code, modify it, redistribute it, etc. Without that element of permission, the code cannot be "Open Source" even if you can physically read it. At best, it might be "Fair Use" depending on the circumstances, but it's most likely just a fancy means of copyright infringement.
Remember, copyright is a legal construct, not a technical one. It depends much more on the intent of the human doing the copying than it does on the technical details of what they actually did. If the thing they have is obviously intended to be a copy of something else, it is a Derivative Work no matter what technical means were used to create it. That means the original copyright still attaches to it and the person who made the copy doesn't get to choose a new license for it, "Open Source" or otherwise.
Why YSK:
You don't have to like the way copyright law works -- I sure don't! -- but you do have to understand it because there's a lot of misinformation going around right now with people claiming things are "Open Source" when they aren't and a lot of people are going to get in trouble for it. It also dilutes the public understanding of what actual legitimate Open Source software is, which is a problem in and of itself.
Conflating real Open Source software with proprietary software that's been 'pirated with extra steps' is harmful both for developers of the former, who have their reputations damaged by association, and for users/sharers of the latter, who might be misled into not taking the same precautions that they would if they understood that they were dealing with warez. Just because you might think Big Tech can get away with laundering copyright through LLMs -- and even that remains to be seen -- doesn't mean the little guys can.
TL;DR: Proprietary software cannot become Open Source software by any means except (a) the express consent of the copyright holder or (b) the copyright expiring and the work becoming Public Domain. Whatever technological end-run you think you have around this legal fact, no you don't.
EDIT: dispute over example
In giving that example I was relying on the claim in the linked thread, which comes from this guy on BlueSky. Seems like a lot of people think he's wrong, so maybe that's not a good example after all.
However, there are also things like this, and those are examples I feel very confident in citing because (a) they explicitly call them "decompliations," (b) at least one of them has a LICENSE file that says it's MIT, and (c) there's zero chance Nintendo or Rare or anyone else legitimately gave them permission for it.
footnote
^1^ "Free Software" has essentially the same denotation as "Open Source" -- close enough that every "Free Software" license is also "Open Source" and vice-versa -- but a different connotation. The term "Free Software" tends to get used by people who wish to emphasize the rights of the end-user, while the term "Open Source" tends to get used by people who wish to emphasize that the software is available to be modified.
In the U.S., copyright protections don't extend to functionality, though. So copying even copyrighted code in a manner to replicate functionality is fair use when there's no other way to accomplish the same thing.
For example, if you have a piece of equipment that checks the software on a cartridge for a string of text that says "Produced by or under license from Sega Enterprises Ltd." before running that software, then it's fair use to copy that exact text so that your software can run on that equipment. And it's fair use to reverse engineer and decompile licensed cartridges to see what the bare minimum necessary to make it work.
One way to prove that you didn't copy the software any more than is strictly necessary for functionality is to fully document the functionality, and then have a skilled programmer take the documentation and write new software from scratch, without ever having seen the original software whose function is being copied. That's a "cleanroom implementation." Compaq and other IBM clones built their own BIOS software to implement the exact same functionality of copyrighted IBM code, and created an entire industry of IBM compatible PCs that weren't actually licensed from IBM. Similarly, Google moved Android off of Sun-licensed Java using a cleanroom implementation (when Oracle bought Sun and Google wanted to get away from Larry Ellison's abusive pricing practices).
Ok, so if it's permissible to reverse engineer the code to create documentation of how it works, and then have someone else take that documentation and implement the functionality using new code, how do AI/LLMs fit into this? Can it be said that it's truly a "cleanroom" when the reverse engineering and decompilation functions are done by the same software that converts the decompiled code into documentation in human language, and then is the same software that converts the documentation into newly implemented code? Doesn't quite hit the same way, and I'm not sure the courts would see it the same way.
All of this is a gray area, and people shouldn't confidently predict what the courts will decide in specific nuanced examples. A lot will depend on the specific details, so there isn't going to be much room for sweeping generalizations.