I would expect:

complete -x -c my_app -a "(__fish_complete_path (commandline --current-token))"

and

complete -F -c my_app

To produce the same results, they differ strongly in capability, as __fish_complete_path does not support:

• globbing i.e. completing src/*/fi to src/*/fish.rs
• non prefix-substrings i.e. completing rc to src
• multi layer path completions i.e. completing sr/sh/fi to src/shell/fish.rs

Obviously it doesn't make sense to use it as a replacement of -F, but I think it should be capable of returning the same matches as you e.g. might want to change the base dir for completions to start.

In case there are any concerns about it changing the way it functions, we could just have an option to enable this or a separate command.