<?php

namespace Customize\Form\Extension;

use Eccube\Form\Type\AddCartType;
use Symfony\Component\Form\AbstractTypeExtension;
use Symfony\Component\Form\Extension\Core\Type\ChoiceType;
use Symfony\Component\Form\FormBuilderInterface;

class AddCartExpandExtension extends AbstractTypeExtension
{
    /**
     * {@inheritdoc}
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        if($builder->has('classcategory_id1')) {
            $options = $builder->get('classcategory_id1')->getOptions();

            // 以下3行を追加する
            $options['compound'] = true;
            $options['expanded'] = true;
            unset($options['choices']['common.select']);

            $builder->add('classcategory_id1', ChoiceType::class, $options);
        }
    }

    /**
     * {@inheritdoc}
     */
    public function getExtendedType()
    {
        return AddCartType::class;
    }
}

Error: Class Customize\Form\Extension\AddCartExpandExtension contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (Symfony\Component\Form\FormTypeExtensionInterface::getExtendedTypes)

public static function getExtendedTypes(): iterable
    {
        return [AddCartType::class];
    }