Identify dependencies when group_depend is involved

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I'm trying to navigate the tree of dependencies of some packages in ROS2 but I'm hitting a possible bug.

For normal build_depend and buildtool_depend I can use ament list_dependencies or inspect the package.xml

However, some packages use group_depend. After reading about the format 3 spec, I see this is kind of a reverse dependency specification. Now I would have to find all packages that state to belong to the group. That seems too impractical to be done by hand. And ament crashes when asking it for dependencies (ROS2 built from sources as of today):

$ ament list_dependencies rosidl_default_generators
Traceback (most recent call last):
  File "/home/jano/opt/ros2/install_isolated/ament_tools/bin/ament", line 11, in <module>
    load_entry_point('ament-tools==0.4.0', 'console_scripts', 'ament')()
  File "/home/jano/opt/ros2/install_isolated/ament_tools/lib/python3.6/site-packages/ament_tools/commands/ament.py", line 88, in main
    rc = args.main(args)
  File "/home/jano/opt/ros2/install_isolated/ament_tools/lib/python3.6/site-packages/ament_tools/verbs/list_dependencies.py", line 97, in main
    g.extract_group_members(packages.values())
  File "/home/jano/opt/ros2/install_isolated/ament_package/lib/python3.6/site-packages/ament_package/group_dependency.py", line 60, in extract_group_members
    assert g.evaluated_condition is not None
AssertionError

Any ideas on how to proceed besides grepping my way forward? Should I report this as a bug or am I doing something wrong? Thanks.