The question Is it illegal to rewrite every line of an open source project in a slightly different way, and use it in a closed source project? makes me wonder what would be considered a clean-room implementation in the era of open source projects.

Hypothetically, if I were to develop a library which duplicates the publicly documented interface of an open-source library, without ever looking at the source code for that library, could that code ever be considered a derivative work?

Obviously it would need the same class hierarchy and method signatures, so that it could be a drop-in replacement - could that in itself, be enough to provoke a copyright claim?

What about if I used the test suite of the open source project to verify whether my clean implementation behaved in the same way as the original library? Would using the test suite be enough to dirty my clean code?

^{As should be expected from a question like this, I am not looking for specific legal advice, but looking to document experiences people may have had with this sort of issue.}