Why is the return type correct if I'm not returning the Mock?

I have this trait which I mix in as needed in my test suites:

trait ValidationMock {

  def createValidationMock(result: Either[Error, Unit] = Right(())): Validation = {
    val validationMock = mock[Validation]

    validationMock.validateEligibility(any[Request], any[String])(any[Context]) returns result
  }
}

This compiles and test run successfully when instantiating this mock in the tests as:

val validationMock = createValidationMock()

But why does it work, if the validationMock mock created in the createValidationMock method is not being returned in the method?

Solution

But why does it work, if the validationMock mock created in the createValidationMock method is not being returned in the method?

Because it is:

mockAction.returns returns ReturnAction[mockAction.type] (see code)
then apply returns ScalaOngoingStubbing[mockAction.type] (see code)
then implicit conversion extracts mock out of that value (see code) - pay attention that [T] in implicit conversion is the mock-type not the mocked call's result type

So all of that is basically

ScalaOngoingStubbing.toMock[Validation](
  new StubbingOps(
    validationMock.validateEligibility(any[Request], any[String])(any[Context])
  ) // this wrapper is probably erased by the macros
    .returns // ReturnAction[Either[Error, Unit]]
    .apply(result) // ScalaOngoingStubbing[Either[Error, Unit]]
) // Validation

I wouldn't be surprised if toMock wasn't just extraction + some asInstanceOf on steroids.