Prevent silent mis-stubbing (#479)

Without this check, stubbing can silently be applied elsewhere.
This fix can only be applied in `KStubbing` as `when` does not accept the mock itself and thus, the issue is presented.

Author: wesalvaro

mockito-kotlin/src/main/kotlin/org/mockito/kotlin/KStubbing.kt

@@ -28,11 +28,12 @@ package org.mockito.kotlin
 import org.mockito.kotlin.internal.createInstance
 import kotlinx.coroutines.runBlocking
 import org.mockito.Mockito
+import org.mockito.exceptions.misusing.NotAMockException
 import org.mockito.stubbing.OngoingStubbing
 import kotlin.reflect.KClass
 
 
-inline fun <T> stubbing(
+inline fun <T : Any> stubbing(
     mock: T,
     stubbing: KStubbing<T>.(T) -> Unit
 ) {
@@ -43,7 +44,10 @@ inline fun <T : Any> T.stub(stubbing: KStubbing<T>.(T) -> Unit): T {
     return apply { KStubbing(this).stubbing(this) }
 }
 
-class KStubbing<out T>(val mock: T) {
+class KStubbing<out T : Any>(val mock: T) {
+    init {
+        if(!mockingDetails(mock).isMock) throw NotAMockException("Stubbing target is not a mock!")
+    }
 
     fun <R> on(methodCall: R): OngoingStubbing<R> = Mockito.`when`(methodCall)
 

mockito-kotlin/src/main/kotlin/org/mockito/kotlin/Spying.kt

@@ -56,7 +56,7 @@ fun <T> spy(value: T): T {
  * Creates a spy of the real object, allowing for immediate stubbing.
  * The spy calls <b>real</b> methods unless they are stubbed.
  */
-inline fun <reified T> spy(value: T, stubbing: KStubbing<T>.(T) -> Unit): T {
+inline fun <reified T : Any> spy(value: T, stubbing: KStubbing<T>.(T) -> Unit): T {
     return spy(value)
         .apply { KStubbing(this).stubbing(this) }!!
 }

tests/src/test/kotlin/test/OngoingStubbingTest.kt

@@ -241,6 +241,16 @@ class OngoingStubbingTest : TestBase() {
         expect(mock.stringResult("B")).toBe("B")
     }
 
+    @Test
+    fun stubbingRealObject() {
+        val notAMock = ""
+
+        /* Expect */
+        expectErrorWithMessage("is not a mock!").on {
+            notAMock.stub { }
+        }
+    }
+
     @Test
     fun stubbingTwiceWithCheckArgumentMatchers_throwsException() {
         /* Expect */