This is an excerpt from the #19677.

The upcoming PHP 7.1 will add support for nullable types. So the function declarations in php 7.1 won't need a default null parameter e.g \StdClass $foo = null but could be written as ?\StdClass $fool.

I'm assuming that there are a lot of argument resolvers e.g controller argument resolver, DI, .. inside a Symfony which would complain or throw an exception on new declaration. The new version will be released around the symfony 3.2 release so this would fit nicely together.