This should not cause a single BC break afaik

You are correct. Technically this is not a BC break. But strictly speaking one could believe the intention was \InvalidArgumentException instead of Symfony\Component\Translation\Exception\InvalidArgumentException

Should we apply this on the lowest supported branch where it goes ?

I think so. But it is your call. That is why I wrote "master"

I think this is more of an "improvement" than a feature or bugfix. Sadly there's no mention of this in the symfony docs/semver. However, I do think it falls under the following:

Coding standard and refactoring: Coding standard fixes or code refactoring are not recommended but can be accepted for consistency with the existing code base, if they are not too invasive, and if merging them on master would not lead to complex branch merging

• https://symfony.com/doc/current/contributing/code/maintenance.html

< 3.2 actually throws \InvalidArgumentException

So for 2.7 you need to update the actual throws as well (https://github.com/symfony/symfony/blob/2.7/src/Symfony/Component/Translation/Translator.php#L463)

Otherwise this could be 3.2 without any BC issue (it's a doc fix, at least for the current implementations ^^).

ref

Thank you @ro0NL. You are correct.

So either merge in master or in 3.2 and up.

Thank you @Nyholm.

Thank you for merging