Fail on mocking sealed subclasses which are final in combination with…

modules/mockk-agent-api/src/jvmMain/kotlin/io/mockk/proxy/common/ProxyMaker.kt

@@ -40,12 +40,12 @@ class ProxyMaker(
         instance: Any?
     ): Cancelable<T> {
 
-        throwIfNotPossibleToProxy(clazz, interfaces)
-
         // Sometimes (e.g. in case of sealed classes) we will create the proxy for a subclass of `clazz` and not `clazz`
         // itself.  We need to determine this early, so that the subclass will be inlined as well.
         val actualClass = findActualClassToBeProxied(clazz)
 
+        throwIfNotPossibleToProxy(actualClass, interfaces)
+
         val cancellation = inline(actualClass)
 
         val result = CancelableResult<T>(cancelBlock = cancellation)

modules/mockk/src/commonTest/kotlin/io/mockk/it/SealedClassMockingTest.kt

@@ -1,9 +1,13 @@
 package io.mockk.it
 
+import io.mockk.MockKException
 import io.mockk.every
 import io.mockk.mockk
+import org.junit.jupiter.api.assertThrows
 import kotlin.test.Test
+import kotlin.test.assertContains
 import kotlin.test.assertEquals
+import kotlin.test.fail
 
 class SealedClassMockingTest {
 
@@ -15,6 +19,18 @@ class SealedClassMockingTest {
         assertEquals(0, base.i)
     }
 
+    @Test
+    fun testActualSealedSubClassCannotBeMockedWithMoreInterfacesBecauseItsFinal() {
+        val exception = assertThrows<MockKException> {
+            mockk<Base>(moreInterfaces = arrayOf(Runnable::class))
+        }
+        exception
+            .cause
+            ?.message
+            ?.also { assertContains(it, "More interfaces requested and class is final") }
+            ?: fail("Missing cause")
+    }
+
     private sealed class Base {
         abstract val i: Int
     }