The Interfaces rule in the official Go Code Review Comments document says that packages should return concrete types rather than interfaces. The motivation for this is so that:
...new methods can be added to implementations without requiring extensive refactoring.
which I accept could be a good thing.
But what if a type I'm writing has a dependency without which it cannot serve its purpose? If I export the concrete type, developers will be able to instantiate instances without that dependency. To code defensively for the missing dependency, I then have to check for it in every method implementation and return errors if it is absent. If the developer missed any hints not to do this in my documentation, she or he won't learn about the problem until run time.
On the other hand, if I declare and return an interface with the methods the client needs, I can unexport the concrete type and enforce the use of a factory method which accepts the dependency as an argument and returns the interface plus an error. This seems like a better way to ensure correct use of the package.
Am I somehow not properly getting into the go spirit by thinking like this? Is the ethic of the language that it's okay to have a less-than-perfect encapsulation to give more flexibility to developers?