// XmlEncoder.php line 159ff
        foreach ($rootNode->attributes as $attrKey => $attr) {
            $data['@'.$attrKey] = $attr->nodeValue;
        }

<?php

declare(strict_types=1);

use Symfony\Component\PropertyInfo\Extractor\ReflectionExtractor;
use Symfony\Component\PropertyInfo\PropertyInfoExtractor;
use Symfony\Component\Serializer\Annotation\SerializedName;
use Symfony\Component\Serializer\Encoder\XmlEncoder;
use Symfony\Component\Serializer\Mapping\ClassDiscriminatorFromClassMetadata;
use Symfony\Component\Serializer\Mapping\Factory\ClassMetadataFactory;
use Symfony\Component\Serializer\Mapping\Loader\AnnotationLoader;
use Symfony\Component\Serializer\Mapping\Loader\LoaderChain;
use Symfony\Component\Serializer\NameConverter\MetadataAwareNameConverter;
use Symfony\Component\Serializer\Normalizer\PropertyNormalizer;
use Symfony\Component\Serializer\Serializer;

require_once 'vendor/autoload.php';

class TestClass
{
    public function __construct(
        #[SerializedName('@aString')]
        public readonly string $aString,
        #[SerializedName('@anInt')]
        public readonly int $anInt
    ) {
    }
}

$metadataFactory = new ClassMetadataFactory(new LoaderChain([new AnnotationLoader(null)]));
$nameConverter = new MetadataAwareNameConverter($metadataFactory);
$propertyTypeExtractor = new PropertyInfoExtractor([], [new ReflectionExtractor()]);
$classDiscriminator = new ClassDiscriminatorFromClassMetadata($metadataFactory);

$serializer = new Serializer(
    [
        new PropertyNormalizer($metadataFactory, $nameConverter, $propertyTypeExtractor, $classDiscriminator)
    ],
    [
        new XmlEncoder()
    ]
);

// this works
var_dump($serializer->deserialize('<outer aString="12345" anInt="12345"></outer>', TestClass::class, 'xml'));

// this also works
var_dump($serializer->deserialize('<outer aString="12345" anInt="12345"> </outer>', TestClass::class, 'xml', ['xml_type_cast_attributes' => false]));

// NotNormalizableValueException: The type of the "aString" attribute for class "TestClass" must be one of "string" ("int" given). 
var_dump($serializer->deserialize('<outer aString="12345" anInt="12345"> </outer>', TestClass::class, 'xml'));