Why does _main_complete not try the next completer when $_comp_mesg is non-zero?

[Marlon Richert <marlon.richert@xxxxxxxxx>]

The following line in _main_complete

[[ -n "$_comp_mesg" ]] && break

has the effect that, whenever _message has been called (with only few
exceptions), the next completer won't be tried, _even when
$compstate[nmatches] is zero._

Why? What is the reason for this?

And is there a convenient way to work around this behavior? I want
_history to be tried when _complete fails, but this behavior often
prevents it. For example, if I try `grep \t`, then I get only the
message `pattern`, whereas I would like to get history words.

And if there is not an easy workaround, would you accept a patch from
me that changes this?

—Marlon