class ObjectOne
{
    /**
     * $foo
     *
     * @var int
     */
    public int $foo;

    public function __construct(int $foo)
    {
        $this->foo = $foo;
    }
}

class ObjectTwo
{
    /**
     * $bar
     *
     * @var int
     */
    public int $bar;

    public function __construct(int $bar)
    {
        $this->bar = $bar;
    }
}

class UnionWrapper
{
    /**
     * $foo
     *
     * @var ObjectOne|ObjectTwo
     */
    public ObjectOne|ObjectTwo $obj;

    public function __construct(ObjectOne|ObjectTwo $obj)
    {
        $this->obj = $obj;
    }

}

$bodyTopLevel = '{
    "foo": 1
  }';

$bodyObject = '{
    "obj": {
        "foo": 1
    }
  }';

$phpDocExtractor = new PhpDocExtractor();
$propertyTypeExtractor = new PropertyInfoExtractor(
    typeExtractors: [$phpDocExtractor],
);
$classMetadataFactory = new ClassMetadataFactory(new AttributeLoader());
$encoders = [new JsonEncoder()];
$normalizers = [
    new ArrayDenormalizer(),
    new ObjectNormalizer(propertyTypeExtractor: $propertyTypeExtractor, classMetadataFactory: $classMetadataFactory),
];

$serializer = new Serializer($normalizers, $encoders);

$responseObject = $serializer->deserialize($bodyObject, 'UnionWrapper', 'json', [AbstractNormalizer::REQUIRE_ALL_PROPERTIES => true]);
var_dump($responseObject);
/*
object(UnionWrapper)#163 (1) {
  ["obj"]=>
  object(ObjectOne)#194 (1) {
    ["foo"]=>
    int(1)
  }
}
*/

$responseTopLevel = $serializer->deserialize($bodyTopLevel, 'ObjectOne|ObjectTwo', 'json', [AbstractNormalizer::REQUIRE_ALL_PROPERTIES => true]);
// Uncaught Symfony\Component\Serializer\Exception\NotNormalizableValueException: Could not denormalize object of type "ObjectOne|ObjectTwo", no supporting normalizer found

final class UnionDenormalizer implements DenormalizerAwareInterface, DenormalizerInterface
{
    use DenormalizerAwareTrait;

    public function setDenormalizer(DenormalizerInterface $denormalizer): void
    {
        $this->denormalizer = $denormalizer;
    }

    public function getSupportedTypes(?string $format): array
    {
        return ['*' => true];
    }

    public function supportsDenormalization(mixed $data, string $type, ?string $format = null, array $context = []): bool
    {
        if ($this->denormalizer === null) {
            throw new \BadMethodCallException(sprintf('The nested denormalizer needs to be set to allow "%s()" to be used.', __METHOD__));
        }
        if (str_contains($type, '|')) {
            $possibleTypes = explode('|', $type);
            $support = true;

            // all possible types must be supported
            foreach ($possibleTypes as $possibleType) {
                $typeSupport = $this->denormalizer->supportsDenormalization($data, $possibleType, $format, $context);
                $support = $support && $typeSupport;
            }
            return $support;
        }

        return false;
    }

    /** @phpstan-ignore-next-line */
    public function denormalize(mixed $data, string $type, ?string $format = null, array $context = []): mixed
    {
    // ...snip...
    }
}