Polymorphism with struct from outside my package

I'm trying to build a method that, using the Kubernetes client-go library, fetches and returns the actual Resources for a given *metav1.OwnerReference. I have this:

func fetchResource(ref *metav1.OwnerReference, options *RequestOptions) (*metav1.ObjectMeta, error) {
	switch ref.Kind {
	case "ReplicaSet":
		return options.Clientset.AppsV1().ReplicaSets(options.Namespace).Get(options.Context, ref.Name, metav1.GetOptions{})
	case "Deployment":
		return options.Clientset.AppsV1().Deployments(options.Namespace).Get(options.Context, ref.Name, metav1.GetOptions{})
	case "Job":
		fallthrough
    // more stuff...
	default:
		return nil, nil
	}
}

This code does not compile because:

> cannot use options.Clientset.AppsV1().ReplicaSets(options.Namespace).Get(options.Context, ref.Name, (metav1.GetOptions literal)) (value of type *"k8s.io/api/apps/v1".ReplicaSet) as *"k8s.io/apimachinery/pkg/apis/meta/v1".ObjectMeta value in return statement

My guess was that since the documentation says that basically all resources embedd the metav1.ObjectMeta, I could use it as a return type.

I tried creating and returning an interface instead, but realized I can't implement it for types outside my package:

type K8sResource interface {
	Name() string
	Kind() string
	OwnerReferences() []metav1.OwnerReference
}

func (pod *corev1.Pod) Name() string {
	return pod.Name
}
func (pod *corev1.Pod) Kind() string {
	return pod.Kind
}
func (pod *corev1.Pod) OwnerReferences() []metav1.OwnerReference {
	return pod.OwnerReferences
}

This code does not compile because:

> invalid receiver *"k8s.io/api/core/v1".Pod (type not defined in this package)

What would be the idiomatic and correct solution here?

If you want to return the imported types as an interface that they don't already implement, you can wrap them in types that do implement it.

For example:

type K8sResource interface {
	Name() string
	Kind() string
	OwnerReferences() []metav1.OwnerReference
}

type replicaSet struct{ *v1.ReplicaSet }

func (s replicaSet) Name() string {
	return s.ReplicaSet.Name
}
func (s replicaSet) Kind() string {
	return s.ReplicaSet.Kind
}
func (s replicaSet) OwnerReferences() []metav1.OwnerReference {
	return s.ReplicaSet.OwnerReferences
}

func fetchResource(ref *metav1.OwnerReference, options *RequestOptions) (K8sResource, error) {
	switch ref.Kind {
	case "ReplicaSet":
		res, err := options.Clientset.AppsV1().ReplicaSets(options.Namespace).Get(options.Context, ref.Name, metav1.GetOptions{})
		if err != nil {
			return nil, err
		}
		return replicaSet{res}, nil // wrap it up
	case "Pod":
		res, err := options.Clientset.AppsV1().Pods(options.Namespace).Get(options.Context, ref.Name, metav1.GetOptions{})
		if err != nil {
			return nil, err
		}
		return pod{res}, nil // wrap it up
	case "Job":
		fallthrough
	// more stuff...
	default:
		return nil, nil
	}
}