[belorn]

You mean the author who wrote his own license to specify how his work may be distributed?

Any person who write their own license to specify in fine details how the work may be used and distributed is going to have a very strong (and likely legal) issue with people who try to circumvent that license. Even a clean room implementation would need source material as instructions, and writing such specification without any copyrighted elements of the source would be quite a challenge when dealing with a very large program that has compatibility to a large ecosystem as its primary feature.