This is an auto-filed bug because sys-cluster/charm calls g++ directly.
The issue was originally discovered on amd64, but it may be reproducible on other arches as well.
If you think that a different summary clarifies the issue better, feel free to change it.
Attached build log and emerge --info.
If you think it doesn't make sense fix these type of issues, I'd like to point out that won't be possible use a different CXX implementation (like clang++) by setting the CXX variable. So this issue has been reproduced by setting the CXX variable to x86_64-pc-linux-gnu-g++ and by removing the /usr/bin/g++ - /usr/bin/c++ binaries.
Created attachment 641828 [details]
build log and emerge --info