It's not limited to similar or identical code. The issue applies to anything 'derived' from copyrighted code. The issue is simply most visible with similar or identical code.
If you have code from an independent origin, this issue doesn't apply. That's how clean room designs bypass copyright. Similarly if the upstream code waives its copyright in certain types of derived works (compiler/runtime exemptions), it doesn't apply.
So if you work on an open source project and learn some techniques from it, and then in your day job you use a similar technique, is that a copyright violation?
Basically does reading GPL code pollute your brain and make it impossible to work for pay later?
If so you should only ever read BSD code, not GPL.
> Basically does reading GPL code pollute your brain and make it impossible to work for pay later?
It seems to me that some people believe it does. Some of the "clean room" projects specifically instructed developers to not even look at GPL code. Specific examples not at hand.
Microsoft appears to believe this (or maybe just MacBU) because I've met employees who tell me they're not allowed to read any public code including Stack Overflow answers.
If you have code from an independent origin, this issue doesn't apply. That's how clean room designs bypass copyright. Similarly if the upstream code waives its copyright in certain types of derived works (compiler/runtime exemptions), it doesn't apply.