/usr/share/bash-completion/completions/addpart is in util-linux 2.27.1-6ubuntu3.
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  | _addpart_module()
{
	local cur
	COMPREPLY=()
	cur="${COMP_WORDS[COMP_CWORD]}"
	case $COMP_CWORD in
		1)
			OPTS="--help --version $(lsblk -pnro name)"
			COMPREPLY=( $(compgen -W "${OPTS[*]}" -- $cur) )
			;;
		2)
			# FIXME: how to determine next free partition number
			;;
		3)
			COMPREPLY=( $(compgen -W "start" -- $cur) )
			;;
		4)
			COMPREPLY=( $(compgen -W "length" -- $cur) )
			;;
	esac
	return 0
}
complete -F _addpart_module addpart
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