[Bug c++/77886] -Wimplicit-fallthrough: breaks duff's device (in function templates)

[marc.mutz at kdab dot com]

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

--- Comment #8 from Marc Mutz <marc.mutz at kdab dot com> ---
#c3 is also in a template, indeed.