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

- hosts: testhost
  gather_facts: false
  vars:
    deep_undefined: "{{ nested_undefined_variable }}"
  tasks:
    - name: These do not throw an error, deep_undefined is just evaluated to undefined, since 2.14
      assert:
        that:
          - lazy_eval or deep_undefined
          - deep_undefined is undefined
          - deep_undefined|default('defaulted') == 'defaulted'
      vars:
        lazy_eval: true

    - name: EXPECTED FAILURE actually using deep_undefined fails
      debug:
        msg: "{{ deep_undefined }}"
      ignore_errors: true
      register: res

    - assert:
        that:
          - res.failed
          - res.msg is contains("'nested_undefined_variable' is undefined")