Assert `isInstanceOf` extensions to improve readablity of JUnit test assertions. I've found this most useful when asserting sealed class result objects as you can asset it's the correct type and then via smart casting continue to assert any specifics of the result, in the sample below I assert the type and then taskId.

AssertExtensions.kt

import org.assertj.core.api.AbstractObjectAssert

@Suppress("UNCHECKED_CAST")
inline fun <reified R> AbstractObjectAssert<*, *>.isNotInstanceOf(): AbstractObjectAssert<*, *>? =
    isNotInstanceOf(R::class.java)

@Suppress("UNCHECKED_CAST")
inline fun <reified R> AbstractObjectAssert<*, *>.isInstanceOf(): AbstractObjectAssert<*, *>? =
    isInstanceOf(R::class.java)

@Suppress("UNCHECKED_CAST")
inline fun <reified R> AbstractObjectAssert<*, *>.isInstanceOf(
    crossinline requirements: (R) -> Unit
): AbstractObjectAssert<*, *>? =
    isInstanceOfSatisfying(R::class.java) { requirements(it) }

SampleTaskTest.kt

import com.nhaarman.mockitokotlin2.any
import com.nhaarman.mockitokotlin2.never
import com.nhaarman.mockitokotlin2.verify
import com.nhaarman.mockitokotlin2.whenever
import com.myapp.challenge.isInstanceOf
import kotlinx.coroutines.test.runTest
import org.assertj.core.api.AssertionsForInterfaceTypes.assertThat
import org.junit.Before
import org.junit.Test
import org.junit.runner.RunWith
import org.mockito.Mock
import org.mockito.junit.MockitoJUnitRunner

@RunWith(MockitoJUnitRunner::class)
class StartTaskUsecaseTest {

    @Mock
    private lateinit var taskRepository: TaskRepository

    private lateinit var sut: StartTaskUsecase

    @Before
    fun setUp() {
        sut = StartTaskUsecase(taskRepository)
    }

    @Test
    fun `invoke SHOULD create new activity WHEN there is not current Activity`() {
        runTest {
            val newTaskId = "123"
            val task = Task(id = newTaskId)
            whenever(taskRepository.newTask()).thenReturn(task)

            val result = sut.invoke()
            
            // use of custom isInstanceOf instead of `isInstanceOf(Success::class.java)`
            assertThat(result).isInstanceOf<Success> {
                assertThat(it.taskId).isEqualTo(newTaskId)
            }
        }
    }
}