在kotlin中，如何模拟封装回调函数

Let's say there's an interface with a callback:

interface SomeInterface {
    fun doSomething(arg: String, callback: (Exception?, Long) -> Unit)
}

which I extend into a suspend function like this:

suspend fun SomeInterface.doSomething(arg: String): Long = suspendCoroutine { cont ->
    this.doSomething(arg) { err, result ->
        if (err == null) {
            cont.resume(result)
        } else {
            cont.resumeWithException(err)
        }
    }
}

I'd like to mock this in tests, but am failing. Ideally I'd like to use something like this:

@Test
fun checkService() {
    runBlocking {
        val myService = mock<SomeInterface>()
        whenever(myService.doSomething(anyString())).thenReturn(1234L)
        val result = myService.doSomething("")
        assertEquals(result, 1234L)
    }
}

上述语法因mockito异常而失败，因为它期待回调使用匹配器.

org.mockito.exceptions.misusing.InvalidUseOfMatchersException: 
Invalid use of argument matchers!
2 matchers expected, 1 recorded:

How can I mock a suspend function like that? If a similar syntax is not possible how can I have the mock call back with the desired arguments such that the suspend variant that is used throughout my code returns the desired result during tests?

Update: It seems it's not possible when it's an extension function. Based on Marko Topolnik's comment, I gather it's because an extension is simply a static function which is out of mockito's capability.

When the suspend function is a member function, then it works as expected, with my original syntax.

以下是一些演示代码的要点: