Details here: https://docs.ansible.com/ansible/devel/porting_guides/porting_guide_6.html#id36
Basically:
```yaml
- name: Prior to 2.13
debug:
msg: '[1] + {{ [2] }}'
- name: 2.13 and forward
debug:
msg: '{{ [1] + [2] }}'
```
Interestingly, we had been using the new/safe syntax in lofs of places.
We were using the broken one in many others though. Hopefully all
instances were fixed by this patch.
Slavi Pantaleev
GoMatrixHosting
Kim Brose
Marko Weltzer
Sabine Laszakovits