The new C++17 language requires g++ >= 8.0. To install it in Ubuntu 18.04:
$ sudo apt install g++-8
...
$ which g++
/usr/bin/g++
$ which g++-8
/usr/bin/g++-8
$ sudo unlink g++
$ sudo link -s /usr/bin/g++-8 /usr/bin/g++
$ g++ --version
g++ (Ubuntu 8.2.0-1ubuntu2~18.04) 8.2.0
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