beginning.scala

case class Foo(id: Key = genKey)
case class Bar(x: String, foo: Key = null, id: Key = genKey)

// in tests
  Bar(testData, testFoo.id, testBarId)

// somewhere in logic
  Bar(sensitiveData, foo.id)

coolBeginning.scala

case class Foo(id: Id[Foo] = genId)
case class Bar(x: String, foo: Id[Foo] = null, id: Id[Bar] = genId)

// in tests
  Bar(testData, testFoo.id, testBarId)

// somewhere in logic
  Bar(sensitiveData, foo.id)

example.txt

scala> import shapeless.TypeOperators._
import shapeless.TypeOperators._

scala> type MyString = Newtype[String, Any]
defined type alias MyString

scala> val x: MyString = newtype("test")
x: MyString = test

scala> x.getClass
res4: java.lang.Class[_ <: AnyRef] = class java.lang.String

id.scala

import shapeless._
import shapeless.TypeOperators._

trait Stub[T] {}
type Id[T] = Newtype[Key, Stub[T]]
def Id[T](s: Key): Id[T] = newtype(s)

mess.scala

case class Foo(id: Key)
case class Baz(id: Key)
case class Bar(x: String, baz: Key = null, foo: Key = null, id: Key = genKey)

// in tests
  Bar(testData, testFoo.id, testBarId) // messed up!

// somewhere in logic
  Bar(sensitiveData, foo.id) // messed up!

noMess.scala

case class Foo(id: Id[Foo])
case class Baz(id: Id[Baz])
case class Bar(x: String, baz: Id[Baz] = null, foo: Id[Bar] = null, id: Id[Bar] = genKey)

// in tests
  Bar(testData, testFoo.id, testBarId) // compile error!

// somewhere in logic
  Bar(sensitiveData, foo.id) // compile error!