Require at least one element in ZF2 FieldSet

The problem

I have a Form and a FieldSet. I would like to validate that the FieldSet is not empty. Also, I want to validate each field in the FieldSet.

So far, whatever I have tried is validating one or the other, but not both. If elements is present in the Form's input filter specification, then it validates that elements is not empty, but does not validate the bar and baz fields of FieldSet. And, of course, the other way around. Any clue as to how to approach this issue would be much appreciated.

The Form

class FooForm extends Form implements InputFilterProviderInterface
{
    public function init()
    {
        $this->add([
            'name'     => 'elements',
            'type'     => Collection::class,
            'required' => true,
            'options'  => [
                'target_element' => [
                    'type' => SomeElementFieldSet::class
                ]
            ]
        ]);
    }

    public function getInputFilterSpecification()
    {
        return [
            [
                'name'        => 'elements',
                'required'    => true,
                'validators'  => [
                    ['name' => 'NotEmpty']
                ]
            ]
        ];
    }
}

The FieldSet

class SomeElementFieldSet extends Fieldset implements InputFilterProviderInterface
{
    public function init()
    {
        $this->add(['name' => 'bar']);
        $this->add(['name' => 'baz']);
    }

    public function getInputFilterSpecification()
    {
        return [
            [
                'name'       => 'bar',
                'required'   => true,
                'validators' => [
                    ['name' => 'NotEmpty']
                ]
            ],
            [
                'name'       => 'baz',
                'required'   => true,
                'validators' => [
                    ['name' => 'NotEmpty']
                ]
            ]
        ];
    }
}

Edit: Added full validation spec.

After getting some hints on Google and digging through the source code, I found a solution. Unfortunately the zend-inputfilter implementation is a little buggy and won't work nicely with getInputFilterSpecification(), but we can just construct our own InputFilter and return that directly:

The Form

class FooForm extends BaseForm
{
    public function init()
    {
        $this->add([
            'name'    => 'elements',
            'type'    => Collection::class,
            'options' => [
                'target_element' => [
                    'type' => SomeElementFieldSet::class
                ]
            ]
        ]);
    }

    public function getInputFilter()
    {
        if (!$this->filter) {
            $this->filter = new InputFilter();

            /** @var Collection $elementsCollection */
            $elementsCollection = $this->fieldsets['elements'];

            /** @var SomeElementFieldSet $elementsFieldSet */
            $elementsFieldSet = $elementsCollection->getTargetElement();

            $collectionFilter = new CollectionInputFilter();
            $collectionFilter->setIsRequired(true);
            $collectionFilter->setInputFilter(
                $elementsFieldSet->getInputFilterSpecification()
            );

            $this->filter->add($collectionFilter, 'elements');
        }

        return $this->filter;
    }
}

This will validate that there is at least one element in the collection. And will validate all the elements one by one by the FieldSet's specification.

One problem persists, though. Whenever the collection is empty, the validation will return false, but will not return any messages. This is due to a bug in the zend-inputfilter component. Issues reported here and here. But that is another problem altogether.