Steps to reproduce:

1. Create a course with several participants.
2. Create 3 groups, "Group A", "Group B" and "Group C".
3. Set "Group C"'s visibility to "Only see own membership"
4. Assign users to each group.
5. Add an assignment to the course page with groups mode set to "Separate Groups"
6. Go into the assignment and click "View all submissions"
   • Expected: The "Separate groups" filter contains all 3 groups
   • Actual: The "Separate groups" filter contains only Group A and Group B.

When developing MDL-68093 this use case was identified, but at the time there was no clear way to have the groups API safely return hidden groups for features such as this, without the risk of exposing them to the user.

With MDL-72397 we added the new $includehidden hidden parameter to group_get_all_groups, to allow specific cases where groups are returned regardless of their visibility, with the responsibility put on the calling code to ensure they are no displayed to unprivileged users. We should be able to make use of this to include hidden groups in the assign module's group filter, when viewing as a teacher who has permission to see these groups.

Some consideration will need to be given into how these groups are listed, to make it clear that this is a non-participation group, so assignments cannot be submitted through the group.