Added test

idea/testData/quickfix/migration/operatorModifier/forOverride.kt

@@ -0,0 +1,10 @@
+// "Add 'operator' modifier" "true"
+open class A {
+    open fun plus(other: A): A = A()
+}
+
+class B : A() {
+    override fun <caret>plus(other: A): A {
+        return super.plus(other)
+    }
+}

idea/testData/quickfix/migration/operatorModifier/forOverride.kt.after

@@ -0,0 +1,10 @@
+// "Add 'operator' modifier" "true"
+open class A {
+    open fun plus(other: A): A = A()
+}
+
+class B : A() {
+    override operator fun <caret>plus(other: A): A {
+        return super.plus(other)
+    }
+}