This is Register-ArgumentCompleter version of https://github.com/psake/psake/blob/master/tabexpansion/PsakeTabExpansion.ps1.

Register-PsakeArgumentCompleter.ps1

$psake = Get-Command -Name Invoke-psake -ErrorAction SilentlyContinue
if ($psake) {
    Register-ArgumentCompleter -Native -CommandName $psake.Name -ScriptBlock {
        param($wordToComplete, $commandAst, $cursorPosition)
        Get-ChildItem "$wordToComplete*.ps1" | Select-Object -ExpandProperty Name
    }

    Register-ArgumentCompleter -CommandName $psake.Name -ParameterName taskList -ScriptBlock {
        param($commandName, $parameterName, $wordToComplete, $commandAst, $fakeBoundParameters)
        if ($commandAst -match '(?<file>[^\.]*\.ps1)') {
            $file = $Matches.file
        }
        else {
            $file = 'psakefile.ps1'
        }
        & $commandName -buildFile $file -docs -nologo | Out-String -Stream | ForEach-Object { if ($_ -match "^[^ ]*") { $matches[0] } } | `
            Where-Object { $_ -notin ('Name', '----', '') } | Where-Object { !$wordToComplete -or $_ -like "$wordToComplete*" }
    }
}