Let's say I wrote a paper which describes & designs a software and its implications, and I license it under the CC BY-SA 4.0. If some else (or me) writes said software, does it need to be in a compatible license (e.g., the GPL v3)?
The license says that derivate works or things made upon said work must be licensed under a compatible license, so I'm wondering if that could apply here.
Copyright protects creative expression, not ideas. To the degree to which the design can be considered a creative expression, the copyleft effects of the CC-BY-SA license would apply and designs that are derived from this design would have to use a compatible license.
But the ideas themselves are not covered by the CC-BY-SA license. Anyone could read the paper and freely implement these ideas in a software, as long as no copyrightable aspects are copied.
What specifically would be protected by copyright would depend on the laws and case law in the relevant jurisdiction. E.g. one jurisdiction might view the design of a class as purely functional and not eligible for copyright, another might see a creative expression in the structure and organization of the API.
