Filtering with needs_variants

[kreuzberger]

I've created a needs_variant like from the examples

needs_variants = {
  "var_a": "'var_a' in sphinx_tags"  # filter_string
}

and added an option to evaluate this variant

needs_variant_options = ["var"]
needs_extra_options = ['var']

I assume to use this now as

.. req:: WOW
  :id: ABC
  :var: var_a

Expected is now that the value of var evaluates to bool.

If i now use this in a filter string, the filter does generate an error:

.. need_table::
  :filter:  "ABC" in id and var

sphinx_needs.api.exceptions.NeedsInvalidFilter:-1: Filter "'ABC' in id and var" not valid. Error: Filter did not evaluate to a boolean, instead <class 'str'>: .

cause it seems that the variant is not evaluated. How could this be evaluated for filtering?