ansible/test/integration/targets/handlers/test_block_as_handler.yml

- hosts: localhost
  gather_facts: false
  tasks:
    - debug:
  handlers:
    - name: handler
      block:
        - name: due to how handlers are implemented, this is correct as it is equivalent to an implicit block
          debug:
        - name: this is a parser error, blocks as handlers are not supported
          block:
            - name: handler in a nested block
              debug: