There is no concept of "clean room" in copyright law
Sure there is. "Clean room" just means you have good legal evidence that you created an original work not a derivative one. It is a bit like having an alibi during a murder trial. There is no law that says you must have an alibi to not be convicted of murder, just like there is no law requiring clean room to dodge the copyright bullet, but having one is strong evidence that you are not guilty.
even APIs (see Oracle vs Google) are not protected by copyright
What I mean is that there is no written law containing the words "clean room", and no requirement that an author has not seen similar works. While "clean room" might be a good defence, there aren't automatically grounds for making an infringement claim just because somebody hasn't followed such a hyper-cautious practice.
Oracle vs Google is complex and I reluctantly chose to gloss over the details. The case still has some appeals to go yet, posssiby to the supreme court. While the difference between APIs being copyrightable vs fair use is important, the practical impact should be negligible: if an API as large as the JDK can be copied without permission, then any API can.
Sure there is. "Clean room" just means you have good legal evidence that you created an original work not a derivative one. It is a bit like having an alibi during a murder trial. There is no law that says you must have an alibi to not be convicted of murder, just like there is no law requiring clean room to dodge the copyright bullet, but having one is strong evidence that you are not guilty.
even APIs (see Oracle vs Google) are not protected by copyright
You need to check the news more. The Federal Circuit Court ruled APIs are copyright able, and handed it back to the lower court to rule on fair use. https://www.eff.org/deeplinks/2014/05/dangerous-ruling-oracl...