#compdef su

local shell comp name usr base expl

[[ $words[2] != - ]]
(( base=$?+2 ))

if [[ CURRENT -eq base ]]; then
  _description expl user
  compgen "$expl[@]" -u && return
  usr=root
elif [[ CURRENT -ge base+1 ]]; then
  usr=$words[base]
else
  return
fi

shell="${${(M@)${(@f)$(</etc/passwd)}:#root*}##*:}"
compset -n $base
for name in $shell $shell:t -default-; do
  comp="$_comps[$name]"
  [[ -n "$comp" ]] && "$comp" && return
done