func (db *actualStoreImplementation) GetTask(t *Task) error {
        if (t.ID != 0) {
            // query by ID, mutate the parameter, return nil
        }
        if (t.Tag != "") {
            // query by tag, mutate the parameter, return nil
        }
        return ErrWhatever
    }

usually I have some other package that defines all of the types that can appear on the wire (which I often call `wire` because `proto` is taken by protobuf), define some exported interface in that package with an unexported method so that no other packages can define new types for that interface, and then have a method on my db structs that returns the wire types, like this:

    func (t Task) Public() wire.Value {
        return wire.Task{
            // explicitly generate what you want
        }
    }