Move: Remove companion object which becomes empty after the move

#KT-13903 Fixed

idea/src/org/jetbrains/kotlin/idea/refactoring/move/moveDeclarations/MoveKotlinDeclarationsProcessor.kt

@@ -54,10 +54,8 @@ import org.jetbrains.kotlin.idea.refactoring.move.moveFilesOrDirectories.MoveKot
 import org.jetbrains.kotlin.idea.refactoring.move.postProcessMoveUsages
 import org.jetbrains.kotlin.idea.references.KtSimpleNameReference.ShorteningMode
 import org.jetbrains.kotlin.idea.search.projectScope
-import org.jetbrains.kotlin.psi.KtClassOrObject
-import org.jetbrains.kotlin.psi.KtElement
-import org.jetbrains.kotlin.psi.KtFile
-import org.jetbrains.kotlin.psi.KtNamedDeclaration
+import org.jetbrains.kotlin.psi.*
+import org.jetbrains.kotlin.psi.psiUtil.containingClassOrObject
 import org.jetbrains.kotlin.psi.psiUtil.getElementTextWithContext
 import org.jetbrains.kotlin.psi.psiUtil.isAncestor
 import org.jetbrains.kotlin.utils.keysToMap
@@ -69,9 +67,17 @@ interface Mover : (KtNamedDeclaration, KtElement) -> KtNamedDeclaration {
         override fun invoke(originalElement: KtNamedDeclaration, targetContainer: KtElement): KtNamedDeclaration {
             return when (targetContainer) {
                 is KtFile -> targetContainer.add(originalElement) as KtNamedDeclaration
-                is KtClassOrObject -> targetContainer.addDeclaration(originalElement) as KtNamedDeclaration
+                is KtClassOrObject -> targetContainer.addDeclaration(originalElement)
                 else -> error("Unexpected element: ${targetContainer.getElementTextWithContext()}")
-            }.apply { originalElement.deleteSingle() }
+            }.apply {
+                val container = originalElement.containingClassOrObject
+                if (container is KtObjectDeclaration && container.isCompanion() && container.declarations.singleOrNull() == originalElement) {
+                    container.deleteSingle()
+                }
+                else {
+                    originalElement.deleteSingle()
+                }
+            }
         }
     }
 

idea/testData/refactoring/move/kotlin/moveNestedClass/dropEmptyCompanion/after/Nested.kt

@@ -0,0 +1,3 @@
+package test
+
+class Nested{}

idea/testData/refactoring/move/kotlin/moveNestedClass/dropEmptyCompanion/after/test.kt

@@ -0,0 +1,4 @@
+package test
+
+class Test1 {
+}

idea/testData/refactoring/move/kotlin/moveNestedClass/dropEmptyCompanion/before/test.kt

@@ -0,0 +1,7 @@
+package test
+
+class Test1 {
+    companion object {
+        class <caret>Nested{}
+    }
+}

idea/testData/refactoring/move/kotlin/moveNestedClass/dropEmptyCompanion/dropEmptyCompanion.test

@@ -0,0 +1,5 @@
+{
+  "mainFile": "test.kt",
+  "type": "MOVE_KOTLIN_NESTED_CLASS",
+  "withRuntime": "true"
+}

idea/tests/org/jetbrains/kotlin/idea/refactoring/move/MoveTestGenerated.java

@@ -311,6 +311,12 @@ public class MoveTestGenerated extends AbstractMoveTest {
         doTest(fileName);
     }
 
+    @TestMetadata("kotlin/moveNestedClass/dropEmptyCompanion/dropEmptyCompanion.test")
+    public void testKotlin_moveNestedClass_dropEmptyCompanion_DropEmptyCompanion() throws Exception {
+        String fileName = KotlinTestUtils.navigationMetadata("idea/testData/refactoring/move/kotlin/moveNestedClass/dropEmptyCompanion/dropEmptyCompanion.test");
+        doTest(fileName);
+    }
+
     @TestMetadata("kotlin/moveNestedClass/innerToTopLevelNoThis/innerToTopLevelNoThis.test")
     public void testKotlin_moveNestedClass_innerToTopLevelNoThis_InnerToTopLevelNoThis() throws Exception {
         String fileName = KotlinTestUtils.navigationMetadata("idea/testData/refactoring/move/kotlin/moveNestedClass/innerToTopLevelNoThis/innerToTopLevelNoThis.test");