fish is a bit insonsistent on it. For instance, `git add <tab>` will only autocomplete for modified files. It will also fill in wildcards, e.g. `cat *.txt <tab>` will expand to show all .txt files. On the failure side, `rm foo <tab>` will still show `foo` as an option.

IME, zsh has better autocompletion (which, at the time at least, was a separate install).