Magento – strict_types declaration must be the very first statement in magento 2.3.1

Ryan H, Thank you for your answer, it helped me to find out the real issue,

I had Magento 2.0.7 code base on my development and Magento 2.1 on my server, so the specific core Magento file looked differnt

namespace Magento\Framework\View\Element\UiComponent\DataProvider;

    use Magento\Framework\Api\Filter;
    use Magento\Framework\Data\Collection;

    interface FilterApplierInterface
    {
        public function apply(Collection $collection, Filter $filter);
    }

This is how it looked like on Magento 2.1, so I changed my module code to be look like this

public function apply(\Magento\Framework\Data\Collection $collection, \Magento\Framework\Api\Filter $filter)
{
        $columns = $this->getFulltextIndexColumns($collection, $collection->getMainTable());
        if (!$columns) {
            return;
        }
        $collection->getSelect()
            ->where(
                'MATCH('.implode(',', $columns).') AGAINST(?)',
                $filter->getValue()
            );
}

and it solved the problem for me.

I don't think Magento 2 is yet compatible with php 7.1-alpha, but this is an alarm signal.
I think you should report this on github.
It could be an issue in the near future,