VisitKtFile.kt

override fun visit(root: KtFile) {
    super.visit(root)

    val classVisitor = UnusedClassVisitor()
    root.accept(classVisitor)

    classVisitor.privateClasses
        .filterNot<KtNamedDeclaration> { it.isUsed() }
        .forEach {
            report(CodeSmell(issue, Entity.from(it), "Private class ${it.nameAsSafeName.identifier} is unused."))
        }
    }
}