Move out of Companion Object: Warn if traget class already contains function with the same signature

#KT-13877 Fixed

idea/src/org/jetbrains/kotlin/idea/intentions/MoveMemberOutOfCompanionObjectIntention.kt

@@ -20,7 +20,12 @@ import com.intellij.openapi.editor.Editor
 import com.intellij.openapi.util.TextRange
 import com.intellij.psi.PsiElement
 import com.intellij.psi.search.searches.ReferencesSearch
+import com.intellij.refactoring.util.RefactoringUIUtil
 import com.intellij.util.containers.MultiMap
+import org.jetbrains.kotlin.descriptors.CallableMemberDescriptor
+import org.jetbrains.kotlin.descriptors.ClassDescriptor
+import org.jetbrains.kotlin.idea.caches.resolve.resolveToDescriptor
+import org.jetbrains.kotlin.idea.codeInsight.DescriptorToSourceUtilsIde
 import org.jetbrains.kotlin.idea.refactoring.checkConflictsInteractively
 import org.jetbrains.kotlin.idea.refactoring.move.moveDeclarations.*
 import org.jetbrains.kotlin.idea.refactoring.runSynchronouslyWithProgress
@@ -28,6 +33,7 @@ import org.jetbrains.kotlin.idea.util.application.runWriteAction
 import org.jetbrains.kotlin.psi.*
 import org.jetbrains.kotlin.psi.psiUtil.containingClassOrObject
 import org.jetbrains.kotlin.psi.psiUtil.isAncestor
+import org.jetbrains.kotlin.util.findCallableMemberBySignature
 
 class MoveMemberOutOfCompanionObjectIntention : SelfTargetingRangeIntention<KtNamedDeclaration>(KtNamedDeclaration::class.java,
                                                                                                 "Move out of companion object") {
@@ -73,6 +79,14 @@ class MoveMemberOutOfCompanionObjectIntention : SelfTargetingRangeIntention<KtNa
             conflicts.putValue(refElement, "Class instance required: ${refElement.text}")
         }
 
+        val targetClassDescriptor = targetClass.resolveToDescriptor() as ClassDescriptor
+        val callableDescriptor = element.resolveToDescriptor() as CallableMemberDescriptor
+        targetClassDescriptor.findCallableMemberBySignature(callableDescriptor)?.let {
+            DescriptorToSourceUtilsIde.getAnyDeclaration(project, it)
+        }?.let {
+            conflicts.putValue(it, "Class '${targetClass.name}' already contains ${RefactoringUIUtil.getDescription(it, false)}")
+        }
+
         project.checkConflictsInteractively(conflicts) {
             runWriteAction {
                 Mover.Default(element, targetClass)

idea/testData/intentions/moveOutOfCompanion/redeclarationConflict.kt

@@ -0,0 +1,7 @@
+// SHOULD_FAIL_WITH: Class 'Test' already contains function f1(Int)
+class Test {
+    fun f1(n: Int){}
+    companion object{
+        fun <caret>f1(n: Int){}
+    }
+}

idea/tests/org/jetbrains/kotlin/idea/intentions/IntentionTestGenerated.java

@@ -9306,6 +9306,12 @@ public class IntentionTestGenerated extends AbstractIntentionTest {
             String fileName = KotlinTestUtils.navigationMetadata("idea/testData/intentions/moveOutOfCompanion/notCompanion2.kt");
             doTest(fileName);
         }
+
+        @TestMetadata("redeclarationConflict.kt")
+        public void testRedeclarationConflict() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("idea/testData/intentions/moveOutOfCompanion/redeclarationConflict.kt");
+            doTest(fileName);
+        }
     }
 
     @TestMetadata("idea/testData/intentions/moveToCompanion")