since you sound knowledgeable- if I go and without any contract "donate" some of my code to the repo- what becomes of the right to that code (the patch I contributed)?
And in turn, the project you submitted it to cannot re-licence that patched section of code (e.g. become either GPL licenced) without your permission, as it does not belong to them.
(Edit for clarity)
[0] https://help.github.com/en/articles/github-terms-of-service#...
Edit: I edited my comment to say “could” instead of “would” because the original author could argue that the author of the patch implicitly gave him the right to redistribute his patch by contributing it to a public repository. I'm not sure it would stand in court, but I'd say it would have a non-null chance of success.
But If the author, who initially claimed the project was a learning project, decided to use it commercially, he clearly wouldn't be allowed to use the patch. (and again, it could be different if the patch author willingly contributed to a commercial product).
The answer is that making the change is already usually copyright infringement (though I think some countries have a concept of private copies being exempt from these types of restrictions). But redistribution of your patch would definitely be copyright infringement because a license to create a derived work was not given to you -- and patches are by definition derived works.