Add test for code using internal compiled incrementally

The issue was fixed during refactoring

    #KT-12923 fixed

libraries/tools/kotlin-gradle-plugin/src/test/kotlin/org/jetbrains/kotlin/gradle/KaptIT.kt

@@ -2,6 +2,7 @@ package org.jetbrains.kotlin.gradle
 
 import org.jetbrains.kotlin.gradle.util.allJavaFiles
 import org.jetbrains.kotlin.gradle.util.getFileByName
+import org.jetbrains.kotlin.gradle.util.modify
 import org.junit.Test
 
 class KaptIT: BaseGradleIT() {
@@ -180,4 +181,24 @@ class KaptIT: BaseGradleIT() {
             assertFileExists("build/classes/main/example/TestClassCustomized.class")
         }
     }
+
+    @Test
+    fun testInternalUserIsModifiedStubsIC() {
+        val options = defaultBuildOptions().copy(incremental = true)
+        val project = Project("kaptStubs", GRADLE_VERSION)
+
+        project.build("build", options = options) {
+            assertSuccessful()
+        }
+
+        val internalDummyUserKt = project.projectDir.getFileByName("InternalDummyUser.kt")
+        internalDummyUserKt.modify { it + " " }
+        val internalDummyTestKt = project.projectDir.getFileByName("InternalDummyTest.kt")
+        internalDummyTestKt.modify { it + " " }
+
+        project.build("build", options = options) {
+            assertSuccessful()
+            assertCompiledKotlinSources(project.relativize(internalDummyUserKt, internalDummyTestKt))
+        }
+    }
 }

libraries/tools/kotlin-gradle-plugin/src/test/resources/testProject/kaptStubs/src/main/java/foo/InternalDummy.kt

@@ -0,0 +1,5 @@
+package foo
+
+internal class InternalDummy {
+    internal val x = "InternalDummy.x"
+}

libraries/tools/kotlin-gradle-plugin/src/test/resources/testProject/kaptStubs/src/main/java/foo/InternalDummyUser.kt

@@ -0,0 +1,7 @@
+package foo
+
+internal class InternalDummyUser {
+    internal fun use(dummy: InternalDummy) {
+        if (dummy.x != "InternalDummy.x") throw AssertionError("dummy.x = ${dummy.x}")
+    }
+}

libraries/tools/kotlin-gradle-plugin/src/test/resources/testProject/kaptStubs/src/test/java/foo/InternalDummyTest.kt

@@ -0,0 +1,12 @@
+package foo
+
+import org.junit.Test
+
+class InternalDummyTest {
+    @Test
+    fun testDummy() {
+        val dummy = InternalDummy()
+        val dummyUser = InternalDummyUser()
+        dummyUser.use(dummy)
+    }
+}