brew_zsh_completion.zsh from Homebrew with the "versions" completion added back in again

_brew.zsh

#compdef brew

# Brew ZSH completion function
# Drop this somewhere in your $fpath (like /usr/share/zsh/site-functions)
# and rename it _brew
#
# altered from _fink

_brew_all_formulae() {
  formulae=(`brew search`)
}

_brew_installed_formulae() {
  installed_formulae=(`brew list`)
}

_brew_installed_taps() {
  installed_taps=(`brew tap`)
}

_brew_outdated_formulae() {
  outdated_formulae=(`brew outdated`)
}

local -a _1st_arguments
_1st_arguments=(
  'cat:display formula file for a formula'
  'cleanup:uninstall unused and old versions of packages'
  'create:create a new formula'
  'deps:list dependencies and dependants of a formula'
  'doctor:audits your installation for common issues'
  'edit:edit a formula'
  'home:visit the homepage of a formula or the brew project'
  'info:information about a formula'
  'install:install a formula'
  'link:link a formula'
  'list:list files in a formula or not-installed formulae'
  'log:git commit log for a formula'
  'missing:check all installed formuale for missing dependencies.'
  'outdated:list formulae for which a newer version is available'
  'pin:pin specified formulae'
  'prune:remove dead links'
  'remove:remove a formula'
  'search:search for a formula (/regex/ or string)'
  'server:start a local web app that lets you browse formulae (requires Sinatra)'
  'tap:tap a new formula repository from GitHub, or list existing taps'
  'unlink:unlink a formula'
  'unpin:unpin specified formulae'
  'untap:remove a tapped repository'
  'update:freshen up links'
  'upgrade:upgrade outdated formulae'
  'uses:show formulae which depend on a formula'
  'versions:show all available formula versions'
)

local expl
local -a formulae installed_formulae installed_taps outdated_formulae

_arguments \
  '(-v)-v[verbose]' \
  '(--cellar)--cellar[brew cellar]' \
  '(--config)--config[brew configuration]' \
  '(--env)--env[brew environment]' \
  '(--repository)--repository[brew repository]' \
  '(--version)--version[version information]' \
  '(--prefix)--prefix[where brew lives on this system]' \
  '(--cache)--cache[brew cache]' \
  '*:: :->subcmds' && return 0

if (( CURRENT == 1 )); then
  _describe -t commands "brew subcommand" _1st_arguments
  return
fi

case "$words[1]" in
  search|-S)
    _arguments \
      '(--macports)--macports[search the macports repository]' \
      '(--fink)--fink[search the fink repository]' ;;
  list|ls)
    _arguments \
      '(--unbrewed)--unbrewed[files in brew --prefix not controlled by brew]' \
      '(--pinned)--pinned[list all versions of pinned formulae]' \
      '(--versions)--versions[list all installed versions of a formula]' \
      '1: :->forms' &&  return 0

      if [[ "$state" == forms ]]; then
        _brew_installed_formulae
        _wanted installed_formulae expl 'installed formulae' compadd -a installed_formulae
      fi ;;
  install|home|homepage|log|info|abv|uses|cat|deps|edit|options|versions)
    _brew_all_formulae
    _wanted formulae expl 'all formulae' compadd -a formulae ;;
  remove|rm|uninstall|unlink|cleanup|link|ln|pin|unpin)
    _brew_installed_formulae
    _wanted installed_formulae expl 'installed formulae' compadd -a installed_formulae ;;
  upgrade)
    _brew_outdated_formulae
    _wanted outdated_formulae expl 'outdated formulae' compadd -a outdated_formulae ;;
  untap)
    _brew_installed_taps
    _wanted installed_taps expl 'installed taps' compadd -a installed_taps ;;
esac