cmdline: how to expand `##` in-place?

Question

In Ex mode, ## can be expanded to the files in the args list when files are expected, see :h cmdline-special

In Ex commands, at places where a file name can be used, the following characters have a special meaning. These can also be used in the expression function expand().

Like !ls ## will expand ## But echo ## will not.

However, if I am going to define my own command, how to tell vim that I want files here so please expand my ##?

Is there a way to expand them in place (like how zsh expands * when you press Tab after it)?

---

An example

command! -nargs=1 Echo :exe "echo '" . <q-args> . "'"

Now I want to run Echo I am here ## with ## expanded.

Answer 1

You could use :h expand() in your command:

command! -nargs=+ Test echo expand("<args>")

This way :Test foo bar will echo foo bar and :Test ## will echo your argument list.

About the in-place expansion as far as I know this is not possible out of the box.

Answer 2

I ended in using cabbr. This is one way.

cnoreabbrev <expr> ##; getcmdtype()==':'?expand('##'):'##;'
cnoreabbrev <expr> #; getcmdtype()==':'?expand('#'):'#;'
cnoreabbrev <expr> %; getcmdtype()==':'?expand('%'):'%;'

Another way is to use cnoremap <expr>

function! s:expand()
  let line = getcmdline()
  let pos = getcmdpos()  " cursor pos
  if pos <= 1
    return ';;'
  endif
  let line = line[0:(pos-2)]
  let match = matchlist(line, '\s\(\(%\|#\|#<\?[0-9]\+\|##\)\(:\S\+\)\?\)$')
  if len(match) < 2 || match[1] == ''
    return ';;'
  else
    return repeat("\<bs>", len(match[1])) . expand(match[1])
  endif
endfunction
cnoremap <expr> ;; <SID>expand()