Please work around it by using gcc-9 for bullseye. I'm not going to cherry-pick single patches from the branch.
As said in https://lists.debian.org/debian-release/2021/03/msg00027.html I'll provide GCC 10.3 for backports.
- Bug#986519: nheko: FTBFS: internal compiler err... Lucas Nussbaum
- Bug#986519: nheko: FTBFS: internal compile... Gianfranco Costamagna
- Processed: Re: nheko: FTBFS: internal comp... Debian Bug Tracking System
- Bug#986519: nheko: FTBFS: internal compile... Matthias Klose
- Bug#986519: nheko: FTBFS: internal compile... Andreas Beckmann
- Bug#986519: nheko: FTBFS: internal com... Hubert Chathi
- Bug#986519: nheko: FTBFS: internal... Andreas Beckmann
- Processed: Re: nheko: FTBFS: internal comp... Debian Bug Tracking System
- Bug#986519: marked as done (nheko: FTBFS: ... Debian Bug Tracking System
