At the moment the AllValidator makes the following extra check:

        if (!is_array($value) && !$value instanceof \Traversable) {
            throw new UnexpectedTypeException($value, 'array or Traversable');
        }

it causes a problem to validate whether a value is an array containing only integers.

Let's see the problem in action. Let's say we have an object that has a field $ids that must be a non-empty array of integers. The naive attempt would be to annotate it like this:

    /**
     * @var int[]
     * @Assert\NotBlank()
     * @Assert\Type("array")
     * @Assert\All({
     *     @Assert\Type("int"),
     *     @Assert\NotBlank(),
     * })
     */
    private $ids;

It works fine, as soon as you pass an array there. But if you pass non-array and non-iterable value, like 42 it fails miserably with the following exception:

Expected argument of type "array or Traversable", "integer" given

Why does the AllValidator throws at all?

        if (null === $value) {
            return;
        }

here it checks whether a value is set at all - so that you could validate it with NotBlank if necessary.

So why does it do the type check with exception, while that part of the job should have been done a Type("array") constraint instead?

What I suggest:

        if (!is_array($value) && !$value instanceof \Traversable) {
            return;
        }

instead. In that case if the value is not an array or traversable - the validation does not run. And if one wants to ensure it's an array - they must do a naive Type("array") check first.