'Mockito.RETURNS_SELF' does not work for a supendable builder methods

[m-koops]

When stubbing a suspendable builder method to return the mock itselfs, this works fine for sync methods, but fails for suspend functions.

The failure can be demonstrated by this simple test:

package test

import com.nhaarman.expect.expect
import kotlinx.coroutines.test.runTest
import org.junit.Test
import org.mockito.Mockito
import org.mockito.kotlin.mock

interface Builder {
    suspend fun builderMethod(): Builder
}

class SimpleTest {
    @Test
    fun `should mock builder method returning mock itself via defaultAnswer`() = runTest {
        /* Given */
        val mock = mock<Builder>(defaultAnswer = Mockito.RETURNS_SELF)

        /* When */
        val result = mock.builderMethod()

        /* Then */
        expect(result).toBeTheSameAs(mock)
    }

    @Test
    fun `should mock builder method returning mock itself via answer`() = runTest {
        /* Given */
        val mock = mock<Builder> {
            onBlocking { builderMethod() } doAnswer Mockito.RETURNS_SELF
        }

        /* When */
        val result = mock.builderMethod()

        /* Then */
        expect(result).toBeTheSameAs(mock)
    }
}