When building =sys-apps/groff-1.21 without a C++ compiler installed, the build fails in the configure step with: checking that C++ compiler can compile simple program... no configure: error: a working C++ compiler is required The package should express a bdep on gcc[cxx], to ensure there is a C++ compiler installed.
we currently do not support depending on a C++ compiler. i don't see any real value in it either. building gcc with USE=-cxx is meant for "power" users doing fine tuning, not general system deployments.