Overridden variable leaking out of dependency declared in role meta?

[Alex Willmer]

I have 2 roles (r1 and r2), r2 depends on r1 and the dependency overrides a variable (x). I would expect the overridden value to only apply while r1 is run as a dependency. However the overridden value gets applied throughout the entire play.

If r1 is run in it's only play, without r2, then I get the value from group_vars/ as I expect.

Below is a minimal reproduction. Have I found a bug, or am I misunderstanding how Ansible variables are scoped?

With thanks, Alex

# group_vars/g1

x: set by group_vars

# roles/r1/tasks/main.yml
- debug:
    var: x

# roles/r2/meta/main.yml

dependencies:
  - role: r1
    vars:
      x: overridden by dependency

# roles/r2/tasks/main.yml

- debug:
    var: x

# inventory

g1:
  hosts:
    localhost:

# playbook.yml

- hosts: g1
  connection: local
  roles:
    - r1
    - r2

# ansible.cfg
[defaults]
stdout_callback=yaml

:/tmp/test [v]$ ansible-playbook -i inventory playbook.yml

PLAY [Play that includes dependency] ******************************************************

TASK [Gathering Facts] ********************************************************************
ok: [localhost]

TASK [r1 : debug] *************************************************************************
ok: [localhost] =>
  x: overridden by dependency

TASK [r1 : debug] *************************************************************************
ok: [localhost] =>
  x: overridden by dependency

TASK [r2 : debug] *************************************************************************
ok: [localhost] =>
  x: overridden by dependency

PLAY [Play that excludes dependency] ******************************************************

TASK [Gathering Facts] ********************************************************************
ok: [localhost]

TASK [r1 : debug] *************************************************************************
ok: [localhost] =>
  x: set by group_vars

PLAY RECAP ********************************************************************************
localhost                  : ok=6    changed=0    unreachable=0    failed=0   

:/tmp/test [v]$ ansible-playbook --version
ansible-playbook 2.7.6
  config file = /tmp/test/ansible.cfg
  configured module search path = [u'/home/alex/.ansible/plugins/modules', u'/usr/share/ansible/plugins/modules']
  ansible python module location = /tmp/v/local/lib/python2.7/site-packages/ansible
  executable location = /tmp/v/bin/ansible-playbook
  python version = 2.7.15rc1 (default, Nov 12 2018, 14:31:15) [GCC 7.3.0]