Demonstration of error verifying calls to a lambda that returns `Unit`

MockkUnitLambdaTest.kt

import io.mockk.mockk
import io.mockk.verify
import org.junit.jupiter.api.Test

class MockkUnitLambdaTest {

  @Test
  fun mockUnitLambda() {
    val fn: (String) -> Unit = mockk(relaxUnitFun = true)

    fn("foo")

    verify { fn("foo") }
  }

  @Test
  fun mockUnitLambdaInvoke() {
    val fn: (String) -> Unit = mockk(relaxUnitFun = true)

    fn.invoke("foo")

    verify { fn.invoke("foo") }
  }

  @Test
  fun mockUnitLambdaRelaxed() {
    val fn: (String) -> Unit = mockk(relaxed = true)

    fn("foo")

    verify { fn("foo") }
  }

}

The first 2 tests fail with io.mockk.MockKException: no answer found for: Function1(#1).invoke(foo). The final test passes because it uses relaxed = true.