[adriaanm]

Thanks for the example. We agree that this is a tricky part in the collections. For Scala 2.13, our headline feature will be simplifying/refining the collections API. One of the areas we'd like to improve is to make the transitions from eager to lazy more clear. Also, we'd like to keep immutable (more) separate from mutable collections. Once we switch gears from 2.12.0 to 2.13 milestones, we'll be sure to provide a convenient way to share gotchas like this, so we can see how to prevent them.