Thank you @nicolas-grekas.

@nicolas-grekas After this change was merged, when running our test suite for the liip/imagine-bundle package using twig@2.x-dev, a PHP error is being thrown with the following message when defining a custom extension that returns a single Twig filter (nothing special about it).

Cannot declare class Twig\TwigFilter, because the name is already in use

We are referencing TwigFilter using the Twig_SimpleFilter class name, but I can also confirm that when using the new namespaced class name Twig\TwigFilter the error is still thrown. Downgrading to 1.x-dev fixes the issue so it appears only the 2.x branch contains this problem.

Removing the following line from lib/Twig/SimpleFilter.php fixes this issue.

class_alias('Twig_SimpleFilter', 'Twig\TwigFilter', false);

But it I'm not sure it's that simple. Based on a cursory examination, it appears like Twig/SimpleFilter.php:23 and Twig/Filter.php:142 are the cause of the error, because one includes the other and they both declare an alias for Twig\TwigFilter. Not positive on how to overcome this, but it doesn't appear they can both alias the same class without a class exists check or another mechanism in place.

If you'd like me to open a proper issue instead of bringing this up on a closed PR, let me know and I'll make it happen.

Also, it seems odd that the test suite for this bundle didn't fail; shouldn't there be unit tests somewhere instantiating a filter to test it can be instantiated?

Please don't comment on closed PR, it makes tracking harder for everyone.
Can you please check #2495 a report there, or open an issue here if more sensible to you?