{{ url('product_detail', {id : Product.id} ) }}

<?php
namespace Customize\Twig;

use Eccube\Entity\Product;
use Eccube\Repository\ProductRepository;
use Symfony\Component\Routing\Generator\UrlGeneratorInterface;

/**
 * Class RoutingExtension
 * @package Customize\Twig
 */
class RoutingExtension extends \Symfony\Bridge\Twig\RoutingExtension
{
    /**
     * @var ProductRepository
     */
    private $productRepository;
    /**
     * RoutingExtension constructor.
     * @param UrlGeneratorInterface $generator
     * @param ProductRepository $productRepository
     */
    public function __construct(UrlGeneratorInterface $generator, ProductRepository $productRepository)
    {
        parent::__construct($generator);

        $this->productRepository = $productRepository;
    }

    /**
     * スラッグを持っている場合はスラッグのURLを生成、
     * スラッグを持っていない場合はIDのURLを生成
     *
     * @param string $name
     * @param array $parameters
     * @param false $schemeRelative
     * @return string
     */
    public function getUrl($name, $parameters = [], $schemeRelative = false)
    {
        if ($name === "product_detail") {
            /** @var Product $product */
            $product = $this->productRepository->find($parameters["id"]);
            if($product->getCartKeyCd()) {
                // スラッグを持っている商品はスラッグのURLを生成
                return parent::getUrl("product_detail", ["id" => $product->getCartKeyCd()], $schemeRelative);
            } else {
                // スラッグを持っていない商品は商品IDのURLを生成
                return parent::getUrl("product_detail", ["id" => $parameters["id"]], $schemeRelative);
            }
        }
        return parent::getUrl($name, $parameters, $schemeRelative);
    }
}

.envのAPP_DEBUG=0

http://localhost/products/detail/*slug*

http://localhost/products/detail/*product_id*