Probably easier to just use an example here. I have a function ss that just calls:
sudo systemctl $argv
Tab completion works fine with the full command, but not for the ss function. I.e.,
ss start mys<tab>
tries to complete from the filesystem rather than using the completion function for systemctl command.
After some reseach, it seems that there are two upcoming features in fish, neither of which is in the current stable release (2.1.1).
The first is a --wraps argument to function and complete, which enables completion inheritance. function foo --wraps=bar gives foo all completions registered for bar.
The second is abbreviations through the abbr command, which work much like vim abbreviations. abbr -a gco='git checkout' registers an abbreviation gco to expand to git checkout. Typing gco followed by a space or enter expands the abbreviation, which naturally allows for any registered completion to work.
Without either of these features, the only remaining thing to do is to write a custom completion script for your function/alias, which is silly.
