The reason is the requirement of c++11 and c++14 which gcc-6.4 should support - and it is stable for 13.0 and 17.0 profile. Is it possible to unmask it for 13.0 profile?
Please switch to a supported profile. >=dev-libs/icu-59 is masked on purpose on 13.0.
I can't switch to 17.0 before April. I need a working machine and don't have the time to emerge -e @world. I already unmasked icu but still thought it would be better to report the issue.