I have PS aliases for git, defined as such:

function git-branch-delete { git branch -D $args }
Set-Alias gbd git-branch-delete

function git-checkout { git checkout $args }
Set-Alias gco git-checkout

The $args represents a branch name, which posh-git would typically autocomplete. However, since aliases are not passed down to posh-git, autocompletion doesn't work.

I monkey-patched this functionality myself, using the technique described in this SO answer:

function TabExpansion($line, $lastWord) {
    # aliases that have some form of tab autocomplete functionality
    # NOTE: ADDED POST-INSTALL BY USER
    $line = $line -replace '^gco ', 'git checkout '
    $line = $line -replace '^gbd ', 'git branch '
    # end aliases

    # rest of the function...
}

It'd be super rad if posh-git could somehow retrieve existing PS aliases related to git functionality like this and patch it into posh-git's autocompletion functionality.

I prefer to use PS aliases over git aliases, primarily for brevity - I'm able to do gcm instead of git co or similar.