programmable completion: completing filenames with default Readline behavior

[Jeremy Lin]

I'm writing a completion where, in some cases, I'd like to use
COMPREPLY=() to indicate that no more arguments to a command are
expected, but in other cases, I'd like to offer the default Readline
behavior for filename completions.

So, if I have a directory 'foo', I'd like the shell to first complete
with 'foo/' and then offer to complete files under 'foo'. But if I do
something like COMPREPLY=($(compgen -f -- "$cur")), then I simply get
a completion of 'foo' (with a space appended).

I know I can pass "-o default" to get Readline's default filename
completion when COMPREPLY is empty, but then that seems to preclude
using COMPREPLY=() to deny completions.

I've tried a number of different workarounds, none of which I was
completely happy with (or I probably wouldn't be asking this
question). The closest I got was setting "-o default", and then using
COMPREPLY=("") to semi-deny completion, and COMPREPLY=() to get
filename completion.

Can anyone offer a better approach?

Thanks,
Jeremy