Testing infrastructure for F18

Beyond compiler support of C++17 features, f18 requires library support too, for, for example, std::variant, which became available in GCC 7.1.

https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2017

https://libcxx.llvm.org/cxx1z_status.html

  • Steve