ansible with uninitialized vars

uninitialized-vars.yaml

---
- hosts: all
  gather_facts: False
  vars:
    foo: "one"
    bar: "{{tbd}}"
  tasks:
  # this first line will fail, saying: fatal: [127.0.0.1] => One or more undefined variables: 'tbd' is undefined
  - debug: msg="{{vars['foo']|default('nope')}}"
  # but these would have been fine
  - set_fact: tbd="two"
  - debug: msg="{{vars['foo']|default('nope')}}"
  - debug: msg="{{vars['doesnotexist']|default('nope')}}"