Upgrade gcc version in Ubuntu 18,04
Solution 1:
filesystem is experimental in g++-7.5.0.
You should still be able to use it by including it as <experimental/filesystem> (which provides namespace std::experimental::filesystem), and linking libstdc++fs explicitly (with -lstdc++fs on the g++ command line), as described in Filesystem in C++17
Alternatively, you can install g++-9 (or even gcc-10) in bionic from the toolchain-r PPA.