Template Parts not displaying when inside patterns

[scruffian]

What problem does this address?

When a template part is inside a pattern block, it appears missing on the frontend. To reproduce:

1. Update to the latest version of WordPress trunk
2. Switch to the TT4 theme
3. Open the site in the frontend and you'll see this error:
   Template part has been deleted or is unavailable: header

What is your proposed solution?

This seems to have been introduced in #54651

The problem seems to be that although the template parts are given theme slugs when they are processed by traverse_and_serialize_blocks, when the block code itself comes to process the block the theme slug is missing. I can't work out why.

cc @gziolo as you worked on #54651 :)

[gziolo]

@ockham, it is very likely that we missed adding the replacement for _inject_theme_attribute_in_block_template_content for patterns as they can also contain template parts.

The way to validate that would be to replace the following code:

https://github.com/WordPress/wordpress-develop/blob/33d124299255e7f7faa3f2dbb5f1c6bb43f9b0f4/src/wp-includes/class-wp-block-patterns-registry.php#L167-L172

to look more like:

https://github.com/WordPress/wordpress-develop/blob/33d124299255e7f7faa3f2dbb5f1c6bb43f9b0f4/src/wp-includes/block-template-utils.php#L552-L560

[scruffian]

It works! WordPress/wordpress-develop#5450

[t-hamano]

It looks like an error similar to this issue has been reported. However, these issues were reported a long time ago and may not be directly related...

• Template part has been deleted or is unavailable #44243
• Render wp:template-part in wp:pattern not working #37128

[gziolo]

@t-hamano, thank you for posting references to other tickets. It would be great to test them with all the latest changes as it seems that they could get resolved, too.

[mikachan]

I believe this has been fixed with WordPress/wordpress-develop#5450, so we can probably close this.