Unused symbol: don't report for type parameter in open class #KT-23639 Fixed

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

@@ -251,6 +251,17 @@ class UnusedSymbolInspection : AbstractKotlinInspection() {
             }
         } else useScope
 
+        if (declaration is KtTypeParameter) {
+            val containingClass = declaration.containingClass()
+            if (containingClass != null) {
+                val isOpenClass = containingClass.isInterface()
+                        || containingClass.hasModifier(KtTokens.ABSTRACT_KEYWORD)
+                        || containingClass.hasModifier(KtTokens.SEALED_KEYWORD)
+                        || containingClass.hasModifier(KtTokens.OPEN_KEYWORD)
+                if (isOpenClass && hasOverrides(containingClass, restrictedScope)) return true
+            }
+        }
+
         return (declaration is KtObjectDeclaration && declaration.isCompanion() &&
                 declaration.getBody()?.declarations?.isNotEmpty() == true) ||
                 hasReferences(declaration, descriptor, restrictedScope) ||

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

@@ -16,4 +16,40 @@
     <problem_class severity="WARNING" attribute_key="NOT_USED_ELEMENT_ATTRIBUTES">Unused Symbol</problem_class>
     <description>Type parameter 'T' is never used</description>
   </problem>
+
+  <problem>
+    <file>openClassTypeParameter.kt</file>
+    <line>1</line>
+    <module>light_idea_test_case</module>
+    <entry_point TYPE="file" FQNAME="temp:///src/openClassTypeParameter.kt" />
+    <problem_class severity="WARNING" attribute_key="NOT_USED_ELEMENT_ATTRIBUTES">Unused Symbol</problem_class>
+    <description>Type parameter 'T' is never used</description>
+  </problem>
+
+  <problem>
+    <file>openClassTypeParameter.kt</file>
+    <line>5</line>
+    <module>light_idea_test_case</module>
+    <entry_point TYPE="file" FQNAME="temp:///src/openClassTypeParameter.kt" />
+    <problem_class severity="WARNING" attribute_key="NOT_USED_ELEMENT_ATTRIBUTES">Unused Symbol</problem_class>
+    <description>Type parameter 'T' is never used</description>
+  </problem>
+
+  <problem>
+    <file>openClassTypeParameter.kt</file>
+    <line>9</line>
+    <module>light_idea_test_case</module>
+    <entry_point TYPE="file" FQNAME="temp:///src/openClassTypeParameter.kt" />
+    <problem_class severity="WARNING" attribute_key="NOT_USED_ELEMENT_ATTRIBUTES">Unused Symbol</problem_class>
+    <description>Type parameter 'T' is never used</description>
+  </problem>
+
+  <problem>
+    <file>openClassTypeParameter.kt</file>
+    <line>13</line>
+    <module>light_idea_test_case</module>
+    <entry_point TYPE="file" FQNAME="temp:///src/openClassTypeParameter.kt" />
+    <problem_class severity="WARNING" attribute_key="NOT_USED_ELEMENT_ATTRIBUTES">Unused Symbol</problem_class>
+    <description>Type parameter 'T' is never used</description>
+  </problem>
 </problems>

idea/testData/inspections/unusedSymbol/typeParameter/openClassTypeParameter.kt

@@ -0,0 +1,33 @@
+interface Interface<T>
+interface Interface2<T>
+class A<T> : Interface2<T>
+
+abstract class Abstract<T>
+abstract class Abstract2<T>
+class B<T> : Abstract2<T>()
+
+sealed class Sealed<T>
+sealed class Sealed2<T>
+class C<T> : Sealed2<T>()
+
+open class Open<T>
+open class Open2<T>
+class D<T> : Open2<T>()
+
+fun main(args: Array<String>) {
+    Interface::class
+    Interface2::class
+    A::class
+
+    Abstract::class
+    Abstract2::class
+    B::class
+
+    Sealed::class
+    Sealed2::class
+    C::class
+
+    Open::class
+    Open2::class
+    D::class
+}