public function returnArg($arg_num = 1)
    {
        $arg_num = $this->assertValidArgNum($arg_num);

        $fqn = $this->function_name->fullyQualifiedName();

        \Patchwork\redefine($fqn, function (...$args) use ($fqn, $arg_num) {
            if ( ! array_key_exists($arg_num - 1, $args)) {
                $count = count($args);
                throw new Exception\InvalidArgumentForStub(
                    "{$fqn} was called with {$count} params, can't return argument \"{$arg_num}\"."
                );
            }

            return $args[$arg_num - 1];
        });
        $this->assertRedefined($fqn);
    }