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##### SUMMARY
Use Ansible 2.8 with a condition using a list variable. Witness deprecation warning:

        [DEPRECATION WARNING]: evaluating [] as a bare variable, this behaviour will go
        away and you might need to add |bool to the expression in the future. Also see
        CONDITIONAL_BARE_VARS configuration toggle.. This feature will be removed in
        version 2.12.

"add |bool" is the wrong thing to do here most likely, because: https://medium.com/opsops/wft-bool-filter-in-ansible-e7e2fd7a148f

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
core?

##### ANSIBLE VERSION
<!--- Paste verbatim output from "ansible --version" between quotes -->
```
ansible 2.8.5
  config file = /home/gert/ansible/ansible.cfg
  configured module search path = ['/home/gert/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /home/gert/.local/share/virtualenvs/ansible-BCbjBVJM/lib/python3.6/site-packages/ansible
  executable location = /home/gert/.local/share/virtualenvs/ansible-BCbjBVJM/bin/ansible
  python version = 3.6.8 (default, Jan 14 2019, 11:02:34) [GCC 8.0.1 20180414 (experimental) [trunk revision 259383]]
```

##### CONFIGURATION
<!--- Paste verbatim output from "ansible-config dump --only-changed" between quotes -->
```
ANSIBLE_PIPELINING(/home/gert/ansible/ansible.cfg) = True
ANSIBLE_SSH_ARGS(/home/gert/ansible/ansible.cfg) = -o ControlMaster=auto
-o ControlPersist=60s
-o StrictHostKeyChecking=yes
-o GlobalKnownHostsFile=./files/openssh-client/ssh_known_hosts
-o UserKnownHostsFile=/dev/null
CACHE_PLUGIN(/home/gert/ansible/ansible.cfg) = jsonfile
CACHE_PLUGIN_CONNECTION(/home/gert/ansible/ansible.cfg) = ~/.ansible/fact_cache
CACHE_PLUGIN_TIMEOUT(/home/gert/ansible/ansible.cfg) = 86400
DEFAULT_GATHERING(/home/gert/ansible/ansible.cfg) = smart
DEFAULT_HOST_LIST(/home/gert/ansible/ansible.cfg) = ['/home/gert/ansible/inventory']
DEFAULT_LOAD_CALLBACK_PLUGINS(/home/gert/ansible/ansible.cfg) = False
DEFAULT_STDOUT_CALLBACK(/home/gert/ansible/ansible.cfg) = yaml
DEFAULT_STRATEGY(/home/gert/ansible/ansible.cfg) = mitogen_linear
DEFAULT_STRATEGY_PLUGIN_PATH(/home/gert/ansible/ansible.cfg) = ['/home/gert/ansible/strategy_plugins']
```

##### OS / ENVIRONMENT
<!--- Provide all relevant information below, e.g. target OS versions, network device firmware, etc. -->


##### STEPS TO REPRODUCE
<!--- Describe exactly how to reproduce the problem, using a minimal test-case -->
Run the playbook below with `ansible-playbook` with version 2.8.x.

<!--- Paste example playbooks or commands between quotes below -->
```yaml
- hosts: localhost
  vars:
    myvar: []
  tasks:
    - debug: msg=foo
      when: myvar
```
Witness the misleading solution to the deprecation warning as depicted in the summary.

<!--- HINT: You can paste gist.github.com links for larger files -->

##### EXPECTED RESULTS
<!--- Describe what you expected to happen when running the steps above -->
I would have expected to see a suggestion about validating the length of the list, e.g.:

" add '| length > 0'"

instead of 

" add |bool"

##### ACTUAL RESULTS
<!--- Describe what actually happened. If possible run with extra verbosity (-vvvv) -->

<!--- Paste verbatim command output between quotes -->
```paste below
ansible-playbook --check test.yml 

PLAY [localhost] ***************************************************************************************************************************************************************************************************************************************************************************************************************************************************

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

TASK [debug] *******************************************************************************************************************************************************************************************************************************************************************************************************************************************************
[DEPRECATION WARNING]: evaluating [] as a bare variable, this behaviour will go away and you might need to add |bool to the expression in the future. Also see CONDITIONAL_BARE_VARS configuration toggle.. This feature will be removed in version 2.12. Deprecation warnings can be disabled by setting deprecation_warnings=False in ansible.cfg.
skipping: [localhost]

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

if isinstance(conditional, bool):
            return conditional
        if templar.is_template(conditional):
            display.warning('conditional statements should not include jinja2 '
                            'templating delimiters such as {{ }} or {%% %%}. '
                            'Found: %s' % conditional)
        bare_vars_warning = False
        if C.CONDITIONAL_BARE_VARS:
            if conditional in all_vars and VALID_VAR_REGEX.match(conditional):
                conditional = all_vars[conditional]
                bare_vars_warning = True
        
        templar.available_variables = all_vars
        try:
            
            disable_lookups = hasattr(conditional, '__UNSAFE__')
            conditional = templar.template(conditional, disable_lookups=disable_lookups)
            if bare_vars_warning and not isinstance(conditional, bool):
                display.deprecated('evaluating %r as a bare variable, this behaviour will go away and you might need to add |bool'
                                   ' to the expression in the future. Also see CONDITIONAL_BARE_VARS configuration toggle' % original,
                                   version="2.12", collection_name='ansible.builtin')
            if not isinstance(conditional, text_type) or conditional == "":
                return conditional
            
            
            disable_lookups |= hasattr(conditional, '__UNSAFE__')
            
            
            class CleansingNodeVisitor(ast.NodeVisitor):