SDC incorrectly throws an exception about embedded slots

I am able to reproduce this.

Test only patch is failing with the expected error: https://git.drupalcode.org/issue/drupal-3446933/-/jobs/1730262#L45

Twig\Error\SyntaxError: An exception has been thrown during the compilation of a template ("We found an unexpected slot that is not declared: [the_slot]. Declare them in "my-banner.component.yml".").

The pipeline with the fix is green.

This is ready to review.

I can reproduce this issue with the Storybook module integration, the patch fixes it, it includes tests, and test-only run is failing.
Code looks good.

The only thing I could suggest is a test with two elements nested instead of one, to check that those are traversed from bottom to top, but that might actually be testing twig itself and not our code.

#2 says we need to consider extend and not just embed but we only add test coverage for embed. Have we considered extend? Should we add a test for it?

@alexpott while working on an example with extends I realized that the trick to forward block contents will not work on extended templates. This is not related to SDC, but a Twig limitation.

{# extends-example.twig #}
{% extends 'sdc_test:just-a-slot' %}
{% set slot_value %}
  {% block nested_slot %}{% endblock %}
{% endset %}

{% block the_slot %}Foo {{ slot_value }}{% endblock %}

In the example above there is no scope for extend-example.twig to grab the contents of the block nested_slot.

Since we are extending another template it does not make sense to introduce a new slot/block, you just use the blocks defined by the parent instead. Therefore I think now, that I was wrong in #2 to suggest that we need to support extends.

Back to RTBC based on the above.

Please ignore my comment in #8 🤦. I was conflating two different issues here.

To recap this issue proves that we can:

1. Have embed within embeds.
2. Have embed within extends.
3. Have embed within embeds and forward blocks from the parent to the children.

If you are curious about my confusion in #8, I was trying to write a test for 4, before I wrote one for 2:

4. Have embed within extends and forward blocks from the parent to the children.

However, that is not valid Twig. And that is what a I was realizing in #8.

The commit above adds test coverage for extends, and we should be good now.