I've ported type_traits from GCC to AVR-GCC with some major modifications to the code structure, style, and used some boost implementations along with some of my own implementations and would like to add my own copyright along with the FSF copyright notice as follows:
Copyright (C) 2020 MyName. Copyright (C) 2007-2020 Free Software Foundation, Inc.
My intentions are to post it on github as a standalone project.
The problem is I don't know if I'm allowed to because on the one hand you have:
Why does the FSF require that contributors to FSF-copyrighted programs assign copyright to the FSF? If I hold copyright on a GPLed program, should I do this, too? If so, how? (#AssignCopyright)
Our lawyers have told us that to be in the best position to enforce the GPL in court against violators, we should keep the copyright status of the program as simple as possible. We do this by asking each contributor to either assign the copyright on contributions to the FSF, or disclaim copyright on contributions.
We also ask individual contributors to get copyright disclaimers from their employers (if any) so that we can be sure those employers won't claim to own the contributions.
Of course, if all the contributors put their code in the public domain, there is no copyright with which to enforce the GPL. So we encourage people to assign copyright on large code contributions, and only put small changes in the public domain.
If you want to make an effort to enforce the GPL on your program, it is probably a good idea for you to follow a similar policy. Please contact firstname.lastname@example.org if you want more information.
On the other hand:
Am I required to claim a copyright on my modifications to a GPL-covered program? (#RequiredToClaimCopyright)
You are not required to claim a copyright on your changes. In most countries, however, that happens automatically by default, so you need to place your changes explicitly in the public domain if you do not want them to be copyrighted.
Whether you claim a copyright on your changes or not, either way you must release the modified version, as a whole, under the GPL (if you release your modified version at all).