unsafe.yml

- hosts: localhost
  gather_facts: no
  vars:
    subexpr: myvar
    powerdns_driver: mysql
    powerdns_config_file: /etc/pdns/pdns.conf
    dict:
      "a{{ subexpr }}-def": "C visible"
      "myvar-b": "{{ subexpr_undef | default('foo') }}"
      "myvar-c": >-
        "{{ subexpr }}"
  tasks:
    - set_fact:
        var2: "{{ dict | combine({}) }}"
    - debug: msg="{{ item.key }} - {{ item.value }}"
      loop: "{{ var2 | dict2items }}"

my output:

ok: [localhost] => (item={'key': u'myvar-c', 'value': u'"myvar"'}) => {
    "msg": "myvar-c - \"myvar\""
}
ok: [localhost] => (item={'key': u'myvar-b', 'value': u'foo'}) => {
    "msg": "myvar-b - foo"
}
ok: [localhost] => (item={'key': u'amyvar-def', 'value': u'C visible'}) => {
    "msg": "amyvar-def - C visible"
}

My broken variation:

ok: [localhost] => (item={'key': u'myvar-c', 'value': u'"myvar"'}) =>
msg: myvar-c - "myvar"
ok: [localhost] => (item={'key': u'myvar-b', 'value': u'foo'}) =>
msg: myvar-b - foo
ok: [localhost] => (item={'key': u'a{{ subexpr }}-def', 'value': u'C visible'}) =>
msg: a{{ subexpr }}-def - C visible