Generic Watches for builder

[pmalek]

Problem statement

#2214 introduced (through #2783) generic funcs for Event, Predicate and Handler.

TypedBuilder has only 1 type parameter for the request type and since Go (as of 1.23) doesn't allow type parameters on methods golang/go#49085, Watches() uses handler.TypedEventHandler[client.Object, request] as eventHandler parameter.

That client.Object limits the handler functions to only have client.Object as argument types so users have to use:

podForService := func(ctx context.Context, obj client.Object) []reconcile.Request {
	svc , ok := obj.(*core.Service)
	if !ok {
		return nil
	}
	return nil
}
mgr, err := ctrl.NewManager(ctrl.GetConfigOrDie(), ctrl.Options{})
b := ctrl.NewControllerManagedBy(mgr)
b.For(&corev1.Pod{})
b.Watches(
	&corev1.Service{},
	handler.TypedEnqueueRequestsFromMapFunc(
		podForService,
	),
)

and cannot use strongly typed handlers like

podForServiceTyped := func(ctx context.Context, svc *corev1.Service) []reconcile.Request {
	return nil
}
mgr, err := ctrl.NewManager(ctrl.GetConfigOrDie(), ctrl.Options{})
b := ctrl.NewControllerManagedBy(mgr)
b.For(&corev1.Pod{})
b.Watches(
	&corev1.Service{},
	handler.TypedEnqueueRequestsFromMapFunc(
		podForServiceTyped,
	),
)

because

cannot use handler.TypedEnqueueRequestsFromMapFunc(podForServiceTyped) (value of type handler.TypedEventHandler[*"k8s.io/api/core/v1".Service, reconcile.Request]) as handler.TypedEventHandler[client.Object, reconcile.Request] value in argument to b.Watches: handler.TypedEventHandler[*"k8s.io/api/core/v1".Service, reconcile.Request] does not implement handler.TypedEventHandler[client.Object, reconcile.Request] (wrong type for method Create)
		have Create(context.Context, event.TypedCreateEvent[*"k8s.io/api/core/v1".Service], workqueue.TypedRateLimitingInterface[reconcile.Request])
		want Create(context.Context, event.TypedCreateEvent[client.Object], workqueue.TypedRateLimitingInterface[reconcile.Request])

Additional information

Due to golang/go#49085 this cannot be (easily?at all?) implemented as noted in #2783.

This issue is mostly for tracking and to acknowledge the API limitation.

Feel free to comment on the issue if you feel there's a way to move forward without Go implementing golang/go#49085.

Current workaround

If I understand correctly, the following is an equivalent which can be used:

src := source.Kind(
	mgr.GetCache(),
	&corev1.Service{},
	handler.TypedEnqueueRequestsFromMapFunc(
		podForServiceTyped,
	),
)
b.WatchesRawSource(src)