/usr/share/fish/completions/id.fish is in fish-common 2.4.0-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 | complete -c id -s g -l group --description "Print effective group id"
complete -c id -s G -l groups --description "Print all group ids"
complete -c id -s n -l name --description "Print name, not number"
complete -c id -s r -l real --description "Print real ID, not effective"
complete -c id -s u -l user --description "Print effective user ID"
complete -c id -l help --description "Display help and exit"
complete -c id -l version --description "Display version and exit"
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