What happened?

Here, the managed.APISimpleReferenceResolver does not consider nil maps & slices as being equal to empty maps or slices. Because in the context of reference resolution, the target referencer fields being set (e.g., Ref fields, if a label selector is specified, and the reference target fields) are semantically equal if they are nil or if they are empty, we had better treat them as equals when doing a comparison. This will prevent unnecessary apply operations when the comparison finds a nil map or slice not equal to an empty one. Using the cmpopts.EquateEmpty() should improve this.

How can we reproduce it?

This behavior can be invoked, for example, by specifying a label selector for a cross-resource reference field when the resolution is not done yet (e.g., the referenced MRs do not exist or not ready yet).

What environment did it happen in?

Crossplane version: 1.14.3