Example for Stackoverflow question https://stackoverflow.com/questions/46234079/in-groovy-spock-assert-call-methods-are-not-executed

MyTest.groovy

trait DiffTrait {

  Javers javers = JaversBuilder.javers().build()

  def result

  def expected

  String diff(result, expected) {
    Diff diff = javers.compare(result, expected);
    def valueChanges = diff.getChangesByType(ValueChange)
    String message = ""
    valueChanges.each { message += "\n$it.propertyName = $it.left instead of expected: $it.right" }
    return message
  }

  String diff() {
    diff(result, expected)
  }

  def result(result) {
    this.result = result
    return result
  }

  def expected(expected) {
    this.expected = expected
    return expected
  }
}

class MyTest extends Specification implements DiffTrait {

    def testFails() {
        when:
        def myResult = new Person(name:"Bob", email:"test@result.org")
        def myExpected = new Person(name:"Bob",  email:"test@expected.org")

        then:
        assert result(myResult) == expected(myExpected), diff()
    }

    def testSucceeds() {
        when:
        def myResult = result(new Person(name:"Bob", email:"test@result.org"))
        def myExpected = expected(new Person(name:"Bob",  email:"test@expected.org"))

        then:
        assert myResult == myExpected, diff()
    }
}

class Person {
    String name
    String email
}