If after installing High Sierra (the latest version of OSX for Macs) you notice issues with Elixir or Phoenix, the fix I’ve found is to update Erlang.
If you installed Erlang with homebrew, you can update by running;
brew upgrade erlang
I’d then suggest forcing a recompile of your project.
mix compile --force
For me the issue presented itself after update when I tried to start my Phoenix server.
It was crashing with the error segmentation fault mix phx.server.
After the Erlang upgrade and the recompile everything was good.