It seems that upstream is working on those GCC-7 errors and warnings. At least they have fixed the -Wimplicit-fallthrough warnings:
https://github.com/dealii/dealii/pull/4230 -- debian-science-maintainers mailing list debian-science-maintainers@lists.alioth.debian.org http://lists.alioth.debian.org/cgi-bin/mailman/listinfo/debian-science-maintainers