How to expand a PowerShell array when passing it to a function

function inner
{
  foreach( $arg in $args )
    {
      # do some stuff
    }
}

function outer
{
  inner --flag $args
}

inner foo bar baz

outer wibble wobble wubble

inner --flag wibble wobble wubble

inner --flag [string]::Join(" ", $args)

function outer
{
    invoke-expression "inner --flag $($args -join ' ')"
}