Let us take this small example with Melody:

<?php
<<<CONFIG
packages:
    - "symfony/serializer: ^3.3"
    - "symfony/property-access: ^3.3"
CONFIG;

$encoders = [new Symfony\Component\Serializer\Encoder\JsonEncoder()];
$normalizers = [new Symfony\Component\Serializer\Normalizer\ObjectNormalizer()];
$serializer = new Symfony\Component\Serializer\Serializer($normalizers, $encoders);

var_dump($serializer->serialize(new class {}, 'json')); // string(2) "[]"

As you can see the result is an empty array ([]) instead of an empty object ({}). IMO this behavior is an important problem and it should be considered as a bug.

This problem comes from the ObjectNormalizer class which returns an empty array so the JsonEncoder class returns an empty JSON array too:

var_dump(json_encode([])); // string(2) "[]"

One solution would be to pass an empty ArrayObject instance to the JsonEncoder class:

var_dump(json_encode(\new ArrayObject)); // string(2) "{}"

To arrive to this result, we have to change the contract of the NormalizerInterface::normalize() method to return an iterable (or a scalable as currently) instead of an array.

In addition, as discussed with @dunglas, this change would allow us to use generators with normalizers.

Then we would be able to adapt the ObjectNormalizer class to deal with an ArrayObject instance instead of an array. As you can imagine, it's a huge BC break but I don't have a better solution to fix this issue...

WDYT?