ansible/test/integration/targets/template/undefined_var_info.yml

- hosts: localhost
  gather_facts: no
  vars:
    foo: []
    bar: "{{ foo[0] }}"
  tasks:
    - debug:
        msg: "{{ bar }}"
      register: result
      ignore_errors: yes

    - assert:
        that:
          - '"foo[0]" in result.msg'
          - '"object has no element 0" in result.msg'