[tricorn]

Copyright protects "authorship", not functionality. Patent protects functional elements.

A rewrite based on functional equivalency is not infringing on the copyright as long as no creative expression was copied. That was the heart of the Google case, whether the API itself was creative expression or functionality.

There are many aspects to what can be considered creative expression, including names, organization, non-functional aspects. An algorithm would not be protected expression. If an AI can write it without reference to the original source code, using only documented behavior, then it would not be infringing (proving that it didn't copy anything from training data might be tough though). It also would not itself be copyrightable, except for elements that could be traced back as "authorship" to the humans who worked with the AI.

If LLMs can create GOOD software based only on functionality, not by copying expression, then they could reproduce every piece of GPL software and release it as Public Domain (which it would have to be if no human has any authorship in it). By the same principle that the GPL software wasn't infringing on the programs they copied functionality from, neither would the AI software. That's a big IF at this point, though, the part about producing GOOD software without copying.