- hosts: testhost
  gather_facts: false
  tasks:
    - name: try to set 'diff' a boolean
      debug: msg="not important"
      diff: yes
      ignore_errors: True
      register: good_diff

    - name: try to set 'diff' a boolean to a string (. would make it non boolean)
      debug: msg="not important"
      diff: yes.
      ignore_errors: True
      register: bad_diff

    - name: Check we did error out
      assert:
        that:
            - good_diff is success
            - bad_diff is failed
            - "'is not a valid boolean' in bad_diff['msg']"