What's the different of <reified T> with <reified T: Any> in Kotlin?

Question

The test class below, pass.

class SimpleClassTest {

    private inline fun <reified T> anyObject(): T {
        return Mockito.anyObject<T>()
    }

    lateinit var simpleObject: SimpleClass
    @Mock lateinit var injectedObject: InjectedClass


    @Before
    fun setUp() {
        MockitoAnnotations.initMocks(this)
    }

    @Test
    fun testSimpleFunction() {
        simpleObject = SimpleClass(injectedObject)
        simpleObject.simpleFunction()

        verify(injectedObject).settingDependentObject(anyObject())
    }
}

But if we change from

    private inline fun <reified T> anyObject(): T {
        return Mockito.anyObject<T>()
    }

to

    private inline fun <reified T: Any> anyObject(): T {
        return Mockito.anyObject<T>()
    }

It will fail with

java.lang.IllegalStateException: Mockito.anyObject<T>() must not be null

What's the different of <reified T> with <reified T: Any> in Kotlin?

UPDATED With the answer that Any is non-null, then using <reified T: Any> shouldn't return error, since settingDependentObject(...) is declared receiving a non-null argument. I would expect <reified T> should error out instead, but it's opposite from what I understand.

Did I understand something wrong?

Answer 1

As stated in the documentation and the linked answer the default upper bound is Any?. In other words the following declarations are equivalent:

inline fun <reified T> anyObject(): T = Mockito.anyObject<T>()
inline fun <reified T:Any?> anyObject(): T = Mockito.anyObject<T>()

The Mockito.anyObject<T>() will return null for both T:Any and T:Any?. When the method with return type T:Any is invoked the null value returned by Mockito fails the runtime check inserted by Kotlin compiler. The error you're getting is thrown before settingDependentObject is invoked.

Answer 2

When you add T : Any constraint on a type parameter, you're making it effectively non-null: T is subtype of Any and Any can't hold nulls.

Since the function is inline and has a reified type parameter, that parameter gets substituted with a real non-nullable type. And thus a null-check is performed on a call site, so you're getting this exception.