[Bug c++/83690] [8 regression] spurious unused variable warings for variables used only in static_assert

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83690

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 43046
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43046&action=edit
gcc8-pr83690.patch

This does seem to do the job.