code: func resourceVSphereComputeClusterHostGroupFlattenID(cluster *object.ClusterComputeResource, name string) (string, error) {
  	clusterID := cluster.Reference().Value
  	return strings.Join([]string{clusterID, name}, ":"), nil
  }
docs: // resourceVSphereComputeClusterHostGroupFlattenID makes an ID for the
  // vsphere_cluster_host_group resource.