think\console\input\Argument::setDefault PHP Method

    public function setDefault($default = null)
    {
        if (self::REQUIRED === $this->mode && null !== $default) {
            throw new \LogicException('Cannot set a default value except for InputArgument::OPTIONAL mode.');
        }
        if ($this->isArray()) {
            if (null === $default) {
                $default = [];
            } elseif (!is_array($default)) {
                throw new \LogicException('A default value for an array argument must be an array.');
            }
        }
        $this->default = $default;
    }