Self referencing dictionary (a property is the result of another) in a Jinja templated file

I'm trying to create an object in which a field depends of another, imagine the group_vars/all file contains something like:

people_names:
 - first_name: tom
   last_name: hardy
   full_name: " {{ first_name}} {{ last_name}} "

The task is very simple, I'm trying to debug this:

- name: Test jinja2template
  template: 
    src: test.j2 
    dest: test.conf

And the test.j2 would be something like:

{% for person in people_names %}
person_full_name = person.full_name
{% endfor %}

Is this even possible in Ansible/Jinja?

To build this I'm running the command:

ansible-playbook jinja2test.yml --check --diff --connection=local

When I run this I get the error AnsibleUndefinedVariable for the last_name.

Q: "Create an object in which a field depends on another."

A: It's not possible. See for example #8603. Create the dictionary with full names if you need it. For example

  vars:
    people_names:
      - first_name: tom
        last_name: hardy
  tasks:
    - set_fact:
        people_full_names: "{{ people_full_names|default([]) +
                               [item|combine({'full_name': full_name})] }}"
      loop: "{{ people_names }}"
      vars:
        full_name: "{{item.first_name}} {{ item.last_name }}"

    - debug:
        var: people_full_names

gives

    "people_full_names": [
        {
            "first_name": "tom", 
            "full_name": "tom hardy", 
            "last_name": "hardy"
        }
    ]