scala-newtype | NewTypes for Scala with no runtime overhead

The README.md of scala-newtype says:

This expands into a type and companion object definition, so newtypes must be defined in an object or package object.

Macros are allowed to expand classes into other classes with the same name and companion objects, but from what I can tell, the newtype annotation turns your case class into an object of the same name (along with a type alias like type DbUrl = DbUrl.Type). This behavior (turning a top-level annottee into a tree of some other kind) isn't allowed. If the annotation had generated a class DbUrl, and maybe an object of the same name, though, it would have been all right, but pretty much anything else won't work.

To fix your problem, all you need to do is move this into a package object (or some other scope, as long as it isn't top-level).

Edit: As Dmytro Mitin pointed out, the created type is not the type of DbUrl but rather something like type DbUrl = DbUrl.Type with an uppercase "T", where the definition of DbUrl.Type looks something like this (I'm just copying this from the README):