/usr/share/fish/completions/pgrep.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 | __fish_complete_pgrep pgrep
complete -c pgrep -s d -r -d 'Output delimiter'
complete -c pgrep -s l -d 'List the process name as well as the process ID'
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