In Ansible, how do I assign a hostvar to a playbook's host?

The directory of my Ansible project has the following structure:

project/
  playbook.yml
  host_vars/
    localhsot.yml
  roles/
    ...
    terraform-provision/
      ...
  terraform/
      ...

Here's project/playbook.yml:

- hosts: localhost
  tasks:
  - name: trigger role
    include_role:
      name: terraform-provision
- hosts: hostvars['localhost'].aws_instance_host_group
  tasks:
  - name: debugging
    debug:
      var: hostvars['localhost'].aws_instance_host_group

Here's the relevant part of project/host_vars/localhost.yml:

hostvars['localhost'].aws_instance_host_group
aws_instance_host_group: "aws_instance"

Here's the relevant part of project/roles/terraform-provision/tasks/main.yml:

- name: Add the provisioned AWS instance to the inventory
  add_host:
    name: "{{ aws_instance_public_ip.value }}" # set using set_fact elsewhere in file
    ansible_user: "{{ aws_instance_user_name }}"
    groups: "{{ aws_instance_host_group }}"

When I execute project/playbook.yml, it throws the following error when trying to connect to the host created by the terraform-provision role:

[WARNING]: Could not match supplied host pattern, ignoring: hostvars['localhost'].aws_instance_host_group

However, the debugging task of project/playbook.yml prints the group's name (aws_instance) successfully.

What am I missing? Is there a better approach to achieve the same thing?

To be evaluated, a variable must be closed in quoted double curly braces, e.g.
- hosts: "{{ hostvars.localhost.aws_instance_host_group }}"


The dot notation simplifies the code.

Minimal reproducible example
shell> cat pb.yml
---

• hosts: localhost

gather_facts: false
tasks:
- set_fact:
aws_instance_host_group: aws_instance
- add_host:
name: 10.1.0.61
groups: "{{ aws_instance_host_group }}"

• hosts: "{{ hostvars.localhost.aws_instance_host_group }}"

gather_facts: false
tasks:
- debug:
var: groups
- debug:
var: inventory_hostname

shell> cat hosts
localhost

shell> ansible-playbook -i hosts pb.yml

PLAY [localhost] ***

TASK [set_fact] ****
ok: [localhost]

TASK [add_host] ****
changed: [localhost]

PLAY [aws_instance] ****

TASK [debug] *
ok: [10.1.0.61] =>
groups:
all:
- 10.1.0.61
- localhost
aws_instance:
- 10.1.0.61
ungrouped:
- localhost

TASK [debug] *
ok: [10.1.0.61] =>
inventory_hostname: 10.1.0.61

PLAY RECAP ***
10.1.0.61: ok=2 changed=0 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0
localhost: ok=2 changed=1 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0


StackExchange DevOps Q#15931, answer score: 2