How do I get bash completion to work with aliases?

Question

As stated in the comments above,

complete -o default -o nospace -F _git_checkout gco

will no longer work. However, there's a __git_complete function in git-completion.bash which can be used to set up completion for aliases like so:

__git_complete gco _git_checkout

I ran into this problem as well and came up with this code snippet. This will automatically give you completion for all aliases. Run it after declaring all (or any) alias.

# wrap_alias takes three arguments:
# $1: The name of the alias
# $2: The command used in the alias
# $3: The arguments in the alias all in one string
# Generate a wrapper completion function (completer) for an alias
# based on the command and the given arguments, if there is a
# completer for the command, and set the wrapper as the completer for
# the alias.
function wrap_alias() {
  [[ "$#" == 3 ]] || return 1

  local alias_name="$1"
  local aliased_command="$2"
  local alias_arguments="$3"
  local num_alias_arguments=$(echo "$alias_arguments" | wc -w)

  # The completion currently being used for the aliased command.
  local completion=$(complete -p $aliased_command 2> /dev/null)

  # Only a completer based on a function can be wrapped so look for -F
  # in the current completion. This check will also catch commands
  # with no completer for which $completion will be empty.
  echo $completion | grep -q -- -F || return 0

  local namespace=alias_completion::

  # Extract the name of the completion function from a string that
  # looks like: something -F function_name something
  # First strip the beginning of the string up to the function name by
  # removing "* -F " from the front.
  local completion_function=${completion##* -F }
  # Then strip " *" from the end, leaving only the function name.
  completion_function=${completion_function%% *}

  # Try to prevent an infinite loop by not wrapping a function
  # generated by this function. This can happen when the user runs
  # this twice for an alias like ls='ls --color=auto' or alias l='ls'
  # and alias ls='l foo'
  [[ "${completion_function#$namespace}" != $completion_function ]] && return 0

  local wrapper_name="${namespace}${alias_name}"

  eval "
function ${wrapper_name}() {
  let COMP_CWORD+=$num_alias_arguments
  args=( \"${alias_arguments}\" )
  COMP_WORDS=( $aliased_command \${args[@]} \${COMP_WORDS[@]:1} )
  $completion_function
  }
"

  # To create the new completion we use the old one with two
  # replacements:
  # 1) Replace the function with the wrapper.
  local new_completion=${completion/-F * /-F $wrapper_name }
  # 2) Replace the command being completed with the alias.
  new_completion="${new_completion% *} $alias_name"

  eval "$new_completion"
}

# For each defined alias, extract the necessary elements and use them
# to call wrap_alias.
eval "$(alias -p | sed -e 's/alias \([^=][^=]*\)='\''\([^ ][^ ]*\) *\(.*\)'\''/wrap_alias \1 \2 '\''\3'\'' /')"

unset wrap_alias

Ideally I'd like autocompletion to just magically work for all my aliases. Is it possible?

Yes, it is possible with the complete-alias project (on Linux). Support for Mac is experimental but users have reported success.

In git-completion.bash there is a line:

complete -o default -o nospace -F _git git

Looking at that line (and the _git function) you can add this line to your .bash_profile:

complete -o default -o nospace -F _git_checkout gco