Problems

In the case you want to deserialize value-objects, if all the data required by its constructor are not given as input, the serializer will throw a simple RuntimeException exception. This makes impossible to catch it. (as current fix on my projects I use exception message to be sure to catch the good one X.x")

The second problem is a missing feature to fill the required object with an empty one. This needs to be defined by the user because the serializer can't guess how to build it.

Here is a project that exposes the problem of the current behavior. https://github.com/Nek-/api-platform-value-object

Solutions suggested

I suggest a solution in 2 parts because the second part is more touchy.

1. Replace the current exception by a new specific one
2. Add a new empty_data option to the context of deserialization so you can specify data for objects impossible to instantiate, this is great because the serializer no more throw exception and the validator can then work as expected and send violations to the user. This solution is inspired by forms solutions to fix the issue with value objects

Here is what you can do with this feature:

class DummyValueObject
{
    public function __construct($foo, $bar) { $this->foo = $foo; $this->bar = $bar; }
}

$empty = new DummyValueObject('', '');
$result = $normalizer->denormalize(['foo' => 'Hello'], DummyValueObject::class, 'json', [
    'empty_data' => [
        DummyValueObject::class => $empty,
    ],
]);

// It's impossible to construct a DummyValueObject with only "foo" value. So the serializer
// will replace it with the given empty data

There are 2 commits so I can quickly provide you only the first point if you want. Hope you'll like this.

Solution after discussion

1. New exception MissingConstructorArgumentsException
2. New context option default_constructor_arguments

class DummyValueObject
{
    public function __construct($foo, $bar) { $this->foo = $foo; $this->bar = $bar; }
}

$result = $normalizer->denormalize(['foo' => 'Hello'], DummyValueObject::class, 'json', [
    'default_constructor_arguments' => [
        DummyValueObject::class => ['foo' => '', 'bar' => ''],
    ],
]);

// DummyValueObject is contructed with the given `foo` and empty `bar`