How to mock a function that returns a value class instance with Mockito?

Question

I'm trying to mock a method that returns an instance of a value class (extends AnyVal) I'm getting some weird error message, which I understand (because of value class erasure) but I'm surprised Mockito doesn't cope with that.

My class:

case class MyValueClass(value: String) extends AnyVal

The function I want to mock:

trait ToMock {
  def something(someParams: String): MyValueClass
}

And the mock:

val theMock = mock[ToMock]
val returned = MyValueClass("test")
when(theMock.something("test")).thenReturn(returned)

This code generates the following error:

MyValueClass cannot be returned by something()
something() should return String

But of course, if I make it return a String, it doesn't compile anymore...

If I remove extends AnyVal, of course it works fine.

Answer 1

OK, I found an answer that works. I need to use the older mockito style of doReturn

doReturn(returned.value).when(theMock).something("test")

Because it's not type-safe, it works. Not fully satisfactory though, as I give up type safety.