Use Ansible Special Variables in Playbook Vars Section

I am trying to create a conditional variable - if the ansible_user is ansible then I will set the ansible_ssh_private_key_file so it doesn't need to be specified on the command line. I have previously used ansible_user within my playbooks, but it appears it doesn't work within the vars section:

---
- hosts: all
  gather_facts: no

  vars:
    private_key_file: "~/.ssh/ansible/ansible_ed25519"
    ansible_ssh_private_key_file: "{{ private_key_file if ansible_user == 'ansible' else '' }}"

  tasks:
    - debug:
        msg: "{{ ansible_ssh_private_key_file }}"

I can reference inventory_hostname in the vars section without a problem, but it doesn't like ansible_user as it results in the following error when running ansible-playbook playbook.yml -i hosts.yml -u ansible -k:

FAILED! => {"msg": "The field 'private_key_file' has an invalid value, which includes an undefined variable. The error was: 'ansible_user' is undefined. 'ansible_user' is undefined. 'ansible_user' is undefined. 'ansible_user' is undefined"}

I am able to query ansible_user in a task further down - anyone know of an alternative to get the -u <user> value from the command line to use in vars?

I have verified that ansible_user is visible within the playbook, e.g:

---
- hosts: all
  gather_facts: no

  tasks:
    - name: "ansible_user"
      debug:
        msg: "{{ ansible_user }}"

If I then run this play I get the following:

ansible-playbook playbook.yml -i hosts.yml -u ansible -k

PLAY [all] **********************************************************************************************************************************************************************

TASK [ansible_user] *************************************************************************************************************************************************************
ok: [icct0-s1-aob-01] => {
    "msg": "ansible"
}

This proves I do have a valid value for ansible_user, but I just can't reference it from within vars.

When setting the ansible user on the command line with -u, the ansible_user variable won't be available as a fact in the header of your play. You can work around this by:

1. Setting ansible_user in your inventory instead of on the command line, or

2. Effectively promoting it to a "real fact" like this:

   - hosts: all
     gather_facts: false
     tasks:
       - set_fact:
           ansible_user: "{{ ansible_user }}"
   
   - hosts: all
   
     vars:
       private_key_file: "~/.ssh/ansible/ansible_ed25519"
       ansible_ssh_private_key_file: "{{ private_key_file if ansible_user == 'ansible' else '' }}"
   
     tasks:
       - debug:
           msg: "{{ ansible_ssh_private_key_file }}"