Use of undefined constants STYLESHEETPATH and TEMPLATEPATH PHP warning

[david-binda]

Describe the bug
The Gutenberg's Global Styles feature is throwing following PHP warnings in curtomizer context on some requests:

Warning: Use of undefined constant STYLESHEETPATH - assumed 'STYLESHEETPATH' (this will throw an Error in a future version of PHP) in /srv/www/wordpress-default/public_html/wp-includes/template.php on line 663
Warning: Use of undefined constant TEMPLATEPATH - assumed 'TEMPLATEPATH' (this will throw an Error in a future version of PHP) in /srv/www/wordpress-default/public_html/wp-includes/template.php on line 666

This is caused by the call of the locate_template in

gutenberg/lib/global-styles.php

Line 14 in 9ce9380

return is_readable( locate_template( 'experimental-theme.json' ) );

which is being hooked to the wp_enqueue_scripts action via some function chain in

gutenberg/lib/global-styles.php

Line 979 in 9ce9380

add_action( 'wp_enqueue_scripts', 'gutenberg_experimental_global_styles_enqueue_assets' );

However, the WordPress customizer may call the wp_enqueue_scripts prior setting up a theme and defining the templating constants ( STYLESHEETPATH, TEMPLATEPATH ) in case a user w/o necessary capabilities for switching theme is attempting to review a customizer changeset. Customizer is hooking the setup_theme action and eventually dying. See:

https://github.com/WordPress/WordPress/blob/a7d6e929c96f2d1e6e0307e781d36c338b8bf877/wp-includes/class-wp-customize-manager.php#L366 and https://github.com/WordPress/WordPress/blob/a7d6e929c96f2d1e6e0307e781d36c338b8bf877/wp-includes/class-wp-customize-manager.php#L505

To reproduce
Steps to reproduce the behavior:

1. Go to a customizer screen of your site under a user account with all appropriate caps needed for making changes in customizer
2. Make any changes ( does not really matter which ones ), but don't publish those
3. Copy the customizer preview iframe URL ( should be something like: ?customize_changeset_uuid=c0af4551-5b8c-4260-ad4e-368af589c737&customize_theme=twentyseventeen&customize_messenger_channel=preview-0 )
4. Open the copied URL while logged in as a subscriber (or any other user w/o switch_themes capability)
5. Notice the PHP warning

Expected behavior
No PHP warning

Editor version (please complete the following information):

• WordPress version: 5.5.1
• Does the website has Gutenberg plugin installed, or is it using the block editor that comes by default? "gutenberg plugin"
• If the Gutenberg plugin is installed, which version is it? 920

[ockham]

This might have been fixed by #27021. I'll defer to @Addison-Stavlo as to whether we can close this issue.

[Addison-Stavlo]

I think this is unrelated. My change didn't touch the functions noted above, also I don't think the function I changed even existed 29 days ago.

is this still happening?

[oandregal]

This was discussed in this thread and there's a path forward to fix it, in case anyone is interested in picking this up.

[oandregal]

I couldn't repro this error. However, in #28942 we're replacing locate_template by something else, so it arguably fixes this.