/usr/share/bash-completion/completions/checkbashisms is in devscripts 2.17.6+deb9u2.
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 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 | # /usr/share/bash-completion/completions/checkbashisms
# Bash command completion for ‘checkbashisms(1)’.
# Documentation: ‘bash(1)’, section “Programmable Completion”.
# Copyright © 2015, Nicholas Bamber <nicholas@periapt.co.uk>
_checkbashisms()
{
    local cur prev words cword special
    _init_completion || return
    if [[ "$cur" == -* ]]; then
        COMPREPLY=( $( compgen -W '--newline --posix --force --extra' -- "$cur" ) )
    else
        COMPREPLY=( $( compgen -o filenames -f -- "$cur" ) )
    fi
    return 0
} &&
complete -F _checkbashisms checkbashisms
# Local variables:
# coding: utf-8
# mode: shell-script
# indent-tabs-mode: nil
# End:
# vim: fileencoding=utf-8 filetype=sh expandtab shiftwidth=4 :
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