for _, p := range policiesToUpdate.ToSlice() {
			if _, found := cache.iamPolicyDocsMap[p]; !found {
				err = errNoSuchPolicy
				return
			}
		}
		newPolicySet = existingPolicySet.Union(policiesToUpdate)
	} else {
		// policies to detach => inputPolicies ∩ existing (intersection)
		policiesToUpdate = policiesToUpdate.Intersection(existingPolicySet)