The copyright argument is a sidetrack both in the PR comment thread and here. The issue opened claims the new code is based on the old code, and therefore derivative, and therefore must be offered in a modified version of the source code under the previous license, LGPL. The complaint is the maintainers violated the terms of LGPL, that they must prove no derivation from the original code to legally claim this is a legal new version without the LGPL license. Claim is if they or Claude read the old code (or of course directly use any of it) it is a license violation. “… in the release 7.0.0, the maintainers claim to have the right to “relicense” the project. They have no such right; doing so is an explicit violation of the LGPL. Licensed code, when modified, must be released under the same LGPL license. Their claim that it is a "complete rewrite" is irrelevant, since they had ample exposure to the originally licensed code (i.e. this is not a "clean room" implementation).“ By this reasoning, I am genuinely asking (I’m not a license expert) if a valid clean room rewrite is possible, because at a minimum you would need a spec describing all behavior, which ses to require ample exposure to the original to be sufficiently precise.

That's not what a derivative work means, though. Being exposed to something doesn't mean you can't create original work which is similar to it (otherwise every song or artwork would be a derivative work of everything before it)

People do cleanroom implementations as a precaution against a lawsuit, but it's not a necessary element.

In fact, even if some parts are similar, it's still not a clear-cut case - the defendant can very well argue that the usage was 1. transformative 2. insubstantial to the entirety of work.

"The complaint is the maintainers violated the terms of LGPL, that they must prove no derivation from the original code to legally claim this is a legal new version without the LGPL license."

The burden of proof is on the accuser.

"I am genuinely asking (I’m not a license expert) if a valid clean room rewrite is possible, because at a minimum you would need a spec describing all behavior, which ses to require ample exposure to the original to be sufficiently precise."

Linux would be illegal if so (they had knowledge of Unix before), and many GNU tools are libre API-compatible reimplementations of previous Unix utilities :)

The copyright argument is the only relevant argument. If the new work is a derived work of the original, then it follows by definition that the new work is under the copryight of the original's author(s). Since the original chardet was distributed by its author(s) only under the LGPL, any copy/derivative of it that anyone else creates must be distributed only under the LGPL, per the terms of the LGPL.

Now, whether chardet 7.0.0 is a derivative of chardet or not is a matter of copyright law that the LGPL has no say on, and a rather murky ground with not that much case law to rely on behind it. If it's not, the new author is free to distribute chardet 7.0.0 under any license they want, since it is a new work under his copyright.