Say a developer has developed a library for their closed-sourced commercial application. Since they want to give back to the open source community, they publish this library under, say, the GPL, but continue to use it in their own application. Since they hold the copyright, that's fine.
Now, a user of the GPL version finds a bug, fixes it and submits a patch to the original developer. As I understand it, to use this bugfix in their closed-source application, the developer needs to have permission from the submitter. If the submitter refuses, the developer has to find another way to fix the bug in the closed-source version.
But what if the bugfix itself is really trivial? Like properly initializing a variable or checking for a null pointer? Something that any half-competent programmer can find and fix in a matter of minutes, given an error description? Is the patch for this still protected by copyright? Or can the original developer implement the identical fix in their closed-source application without consent of the submitter?
Note: This really is a hypothetical scenario, not one of those "my 'friend' has this problem" questions