/usr/share/fish/completions/cupsreject.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 | __fish_complete_lpr cupsreject
complete -c cupsreject -s r -d 'Reject reason' -x
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