CartItemType.php

<?php

namespace AppBundle\Form\Type;

use Sylius\Bundle\CartBundle\Form\Type\CartItemType as BaseCartItemType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;

/**
 * We extend the item form type a bit, to add a variant select field
 * when we're adding product to cart, but not when we edit quantity in cart.
 * We'll use simple option for that, passing the product instance required by
 * variant choice type.
 */
class CartItemType extends BaseCartItemType
{
    /**
     * {@inheritdoc}
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        parent::buildForm($builder, $options);

        if (isset($options['product']) && $options['product']->hasVariants()) {
            $builder->add('variant', 'app_product_variant_match', array(
                'variable'  => $options['product']
            ));
        }
    }

    /**
     * We need to override this method to allow setting 'product'
     * option, by default it will be null so we don't get the variant choice
     * when creating full cart form.
     *
     * {@inheritdoc}
     */
    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        parent::setDefaultOptions($resolver);

        $resolver
            ->setOptional(array(
                'product'
            ))
            ->setAllowedTypes(array(
                'product' => array('Sylius\Component\Core\Model\ProductInterface')
            ))
        ;
    }
}

config.yml

# app/config/config.yml

sylius_cart:
    classes:
        item:
            form: AppBundle\Form\Type\CartItemType

services.xml

<?xml version="1.0" encoding="UTF-8" ?>

<container xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xmlns="http://symfony.com/schema/dic/services"
           xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>
        <service id="form.type.app_product_variant_match" class="AppBundle\Form\Type\VariantMatchType">
            <argument type="string">product</argument>
            <tag name="form.type" alias="app_product_variant_match"/>
        </service>
    </services>

</container>

VariantMatchType.php

<?php


namespace AppBundle\Form\Type;

use Doctrine\Common\Collections\ArrayCollection;
use Gedmo\Sluggable\Util\Urlizer;
use Sylius\Bundle\VariationBundle\Form\DataTransformer\VariantToCombinationTransformer;
use Sylius\Bundle\VariationBundle\Form\Type\VariantMatchType as BaseVariantMatchType;
use Sylius\Component\Core\Model\ProductVariant;
use Sylius\Component\Product\Model\Option;
use Symfony\Component\Form\FormBuilderInterface;

/**
 * Variant match form type.
 */
class VariantMatchType extends BaseVariantMatchType
{
    /**
     * {@inheritdoc}
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        /** @var Option $option */
        foreach ($options['variable']->getOptions() as $i => $option) {
            $oldValues = $option->getValues();
            $option->setValues(new ArrayCollection());

            /** @var ProductVariant $variant */
            foreach ($options['variable']->getVariants() as $variant) {
                foreach ($oldValues as $value) {
                    if($variant->hasOption($value)) {
                        $option->addValue($value);
                    }
                }
            }

            $builder->add(
                Urlizer::urlize($option->getName()),
                sprintf('sylius_%s_option_value_choice', $this->variableName),
                array(
                    'label' => $option->getPresentation(),
                    'option' => $option,
                    'property_path' => '[' . $i . ']'
                )
            );
        }

        $builder->addModelTransformer(new VariantToCombinationTransformer($options['variable']));
    }

    /**
     * {@inheritdoc}
     */
    public function getName()
    {
        return sprintf('app_%s_variant_match', $this->variableName);
    }
}