Symfony version(s) affected: 4.3.8

Description
If "default_index_method" is used and "index_by" is omitted, the tagged services are not indexed.

How to reproduce

1. Use _instanceof in your services.yaml to attach a tag to an interface

_instanceof:
    App\Test\TagTestInterface:
        tags: ['app.list']

2. Add a class to your services.yaml and inject the tagged list:

TestTagList:
    arguments: [!tagged { tag: app.list } ]

3. Use the "default_index_method" as described here (https://symfony.com/doc/current/service_container/service_subscribers_locators.html#indexing-the-collection-of-services)

TestTagList:
    arguments: [!tagged { tag: app.list, default_index_method: 'getTest' } ]

4. The injected tagged list will not have been indexed.

Possible Solution
You can add
index_by: 'somestring'
in order for "default_index_method" to be working.

TestTagList:
    arguments: [!tagged { tag: app.list, index_by: 'somestring', default_index_method: 'getTest' } ]

Maybe this is not a bug but just not clear enough in the docs.