Working with the Symfony serializer gave me quite a few headaches and this problem was no different. However the solution was easy yet I'm uncertain if this is the right approach and really a bug.

I found that nested objects (e.g. User having a Group) had problems deserializing. User deserializing went fine upon the point where it tries to set the group using setGroup(Group $group) which obviously requires a Group argument and it complains that an array was given instead. Also never an attempt was made to deserialize the Group.

So it knew it expected a Group but yet it ignored it and simply went on and then was like "I told you so, stupid.".

After an hour I finally figured out that the PropertyTypeExtractorInterface constructor argument was null even though property_info was enabled in the framework settings.

TL;DR;
An alias for Symfony\Component\PropertyInfo\PropertyTypeExtractorInterface to @property_info is missing from the service definitions in symfony/framework-bundle/Resources/config/property_info.xml

Edit: In hindsight I notice it's argument is given explicitly in the normalizer service definitions. Still I wonder why was chosen for this approach instead of aliasing the interface so custom normalizers can be autowired.

Please let me know if/why was chosen not to add this alias by default and if there is any other way to get the same functionality. I'd be happy to provide a PR.