$classMetadataFactory = new ClassMetadataFactory(new AnnotationLoader(new AnnotationReader()));
$extractor = new PropertyInfoExtractor([], [new PhpDocExtractor(), new ReflectionExtractor()]);
$serializer = new Serializer(
  [
      new ObjectNormalizer($classMetadataFactory, null, null, $extractor, new ClassDiscriminatorFromClassMetadata($classMetadataFactory)),
  ],
  ['json' => new JsonEncoder()]
);

$actual = $serializer->deserialize('{ "v": { "a": 0 }}', DummyUnionWithAAndB::class, 'json', [
  AbstractNormalizer::ALLOW_EXTRA_ATTRIBUTES => false,
]);

$this->assertEquals(new DummyUnionWithAAndB(new DummyATypeForUnion()), $actual);


$actual = $serializer->deserialize('{ "v": { "b": 1 }}', DummyUnionWithAAndB::class, 'json', [
  AbstractNormalizer::ALLOW_EXTRA_ATTRIBUTES => false,
]);

$this->assertEquals(new DummyUnionWithAAndB(new DummyBTypeForUnion()), $actual);


$this->expectException(ExtraAttributesException::class);
$serializer->deserialize('{ "v": { "b": 1, "c": "i am not allowed" }}', DummyUnionWithAAndB::class, 'json', [
  AbstractNormalizer::ALLOW_EXTRA_ATTRIBUTES => false,
]);

class DummyATypeForUnion {
    public $a = 0;
}

class DummyBTypeForUnion {
    public $b = 1;
}

class DummyUnionWithAAndB {
    /** @var DummyATypeForUnion|DummyBTypeForUnion */
    public $v;

    /**
     * @param DummyATypeForUnion|DummyBTypeForUnion $v
     */
    public function __construct($v) { $this->v = $v; }

}