[Bug c/77886] -Wimplicit-fallthrough: breaks duff's device

[marc.mutz at kdab dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77886

--- Comment #5 from Marc Mutz <marc.mutz at kdab dot com> ---
It's in a function template, in case that helps. I'll try to trim it down now.