#compdef su

local shell comp name usr base

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

if [[ CURRENT -eq base ]]; then
  _users && return
  usr=root
elif [[ CURRENT -ge base+1 ]]; then
  usr=$words[base]
else
  return
fi

shell="${${(M@)${(@f)$(</etc/passwd)}:#$usr*}##*:}"
compset -n $base

_dispatch $shell:t $shell $shell:t -default-