magento2: Parse error: syntax error, unexpected '?', expecting variable (T_VARIABLE) - Magento 2.3

Preconditions

• Magento 2.3-alpha57
• PHP 7.1.19
• MySQL 5.7

Steps to reproduce

Magento 2.3 was working, but after upgrading to the latest alpha release Magento has the following error: Parse error: syntax error, unexpected '?', expecting variable (T_VARIABLE) in .../vendor/magento/framework/Filesystem/Directory/Write.php on line 35

There is a question mark before PathValidatorInterface and is added on the 13th of July. https://github.com/magento/magento2/blob/2.3-develop/lib/internal/Magento/Framework/Filesystem/Directory/Write.php

After removing the question mark for testing, I get the same error in …/vendor/magento/framework/Filesystem/Directory/Read.php on line 54 and after removing that one I get the same error on line 71 and so on.