Being completely untainted is the standard many reimplementations set for themselves to completely rule out legal trouble. For example ReactOS won't let you contribute if you have ever seen Windows code. Because if you have never seen it, there can be no allegation that you copied it.

That is however stricter than what's actually legally necessary. It's just that the actual legal standard would require a court ruling to determine if you passed it, and everyone wants to avoid that. As a consequence there also aren't a lot of court cases to draw similarities to

> For example ReactOS won't let you contribute if you have ever seen Windows code. Because if you have never seen it, there can be no allegation that you copied it.

I've heard this called in some circles "The curse of knowledge." The same thing applies to emulator developers, especially N64 developers (and now Nintendo emulator developers in general) after the Oman Archive and later Gigaleaks. There's an informal "If you read this, you can NEVER directly contribute to the development of that emulator, ever."

This comes to a head when a relatively unknown developer starts contributing oddly specific patches to an emulator.

"Taint" requires that the code is demonstratably derivative from the *GPL licensed work.

This is actually harder standard than some people think.

The absolute clean room approaches in USA are there because they help short circuit a long lawsuit where a bigger corp can drag forever until you're broken.

It's harder than some people think, but the author does a lot of the work when he names the resulting artifact "chardet v7.0.0". If I thought I was writing the kind of arms-length reimplementation that's required, I would never put it into the versioning scheme of the original, come on.

it can be API compatible and legally original

not to mention that it's not a complete copy, because it has different behavior (the better performance)

but of course we have to check the code too

Not a lawyer, but that always seemed naively correct to me.

However, the copyright system has always be a sham to protect US capital interests. So I would be very surprised if this is actually ruled/enforced. And in any case american legislators can just change the law.