How to disable certain completion tags?

[Martin Vaeth <martin@mvath.de>]

I have written a completion for a complex command; calculating
completions for some options of this command needs a long time
(and produces a huge list).

Some of these options should share a common tag, and I want that the
user has a way to disable the completion of that tag so that the
long time is not required when the user presses tab accidentally
or if he uses something like https://github.com/hchbaw/auto-fu.zsh/

To my knowledge, it should be possible to use the tag mechanism
to achieve this, but it does not seem to work as I had understood it.

Here is a minimal example of a completion file demonstrating how
I attempt to achieve the goal with the tag name "foos" for the
command "dummy"; the "sleep" should indicate where the command
used in my actual code takes a long time.

#compdef dummy
local curcontext="$curcontext" state state_descr line expl
typeset -A opt_args
_arguments -C : '--foo=[desc]:some foo:->foo'
ret=$?
case $state in
(foo)
  _tags foos
  _tags && {
    _description foos expl 'available foo'
    compadd "$expl[@]" - $(sleep 5; echo foo1 foo2)
  } && ret=0;;
esac
return ret

My understanding is that the second "_tags" should end with nonzero
state if the user has disabled all tags from the list (and the list
should consist only of "foos" due to the first "_tags foos" command).
However, if I (as user) attempt to disable the "foos" tag for the
"dummy" completion with something like

zstyle ':completion:*:*:dummy:*' tag-order ''

it does not show any effect. I think once (some years ago, I was now not
able to reproduce) I had found some zstyle command which disabled the
completion, but nevertheless "dummy foo=<tab>" had taken a long time
to complete (to nothing) with the above code.