KT-7021 Don't mark companion object as unused if some stuff from it is used

#KT-7021 fixed

idea/src/org/jetbrains/kotlin/idea/inspections/UnusedSymbolInspection.kt

@@ -107,9 +107,6 @@ public class UnusedSymbolInspection : AbstractKotlinInspection() {
                 if (declaration is JetParameter && (declaration.getParent()?.getParent() !is JetClass || !declaration.hasValOrVarNode())) return
                 if (declaration is JetNamedFunction && isConventionalName(declaration)) return
 
-                // TODO companion objects are temporarily disabled
-                if (isCompanionObject) return
-
                 // More expensive, resolve-based checks
                 if (isEntryPoint(declaration)) return
                 // properties can be referred by component1/component2, which is too expensive to search, don't mark them as unused
@@ -165,6 +162,7 @@ public class UnusedSymbolInspection : AbstractKotlinInspection() {
         val psiSearchHelper = PsiSearchHelper.SERVICE.getInstance(declaration.getProject())
 
         val useScope = declaration.getUseScope()
+        if (!(declaration is JetObjectDeclaration && declaration.isCompanion())) {
             if (useScope is GlobalSearchScope) {
                 var zeroOccurrences = true
 
@@ -180,9 +178,10 @@ public class UnusedSymbolInspection : AbstractKotlinInspection() {
                     return false
                 }
             }
+        }
 
         val searchHelper: UsagesSearchHelper<out JetNamedDeclaration> = when (declaration) {
-            is JetClass -> ClassUsagesSearchHelper(constructorUsages = true, nonConstructorUsages = true, skipImports = true)
+            is JetClassOrObject -> ClassUsagesSearchHelper(constructorUsages = true, nonConstructorUsages = true, skipImports = true)
             is JetNamedFunction -> FunctionUsagesSearchHelper(skipImports = true)
             is JetProperty, is JetParameter -> PropertyUsagesSearchHelper(skipImports = true)
             else -> DefaultSearchHelper<JetNamedDeclaration>()

idea/testData/inspections/unusedSymbol/object/defaultObjectUsedInContainingClass.kt

@@ -0,0 +1,21 @@
+class A {
+    init {
+        foo()
+        v + v
+
+        v[v] // using companion object function by convention
+    }
+
+    companion object {
+        fun foo() {
+        }
+
+        val v = 42
+
+        fun Int.get(a: Int) = this + a
+    }
+}
+
+fun main(args: Array<String>) {
+    A()
+}

idea/testData/inspections/unusedSymbol/object/inspectionData/expected.xml

@@ -32,7 +32,6 @@
     <description>Object 'A' is never used</description>
   </problem>
 
-<!--
   <problem>
     <file>defaultObjectUnused.kt</file>
     <line>2</line>
@@ -50,5 +49,4 @@
     <problem_class severity="WARNING" attribute_key="NOT_USED_ELEMENT_ATTRIBUTES">Unused Symbol</problem_class>
     <description>Object 'Named' is never used</description>
   </problem>
--->
 </problems>