Couldn't we do that in the extension? And shouldn't we also add the paths discovered from other bundles here too?

I tried to do that in the extension and it works, but I want to keep warming the templates only if the form.extension service is enabled and I cannot check it in the extension because the service isn't available there. But if it isn't big problem with warming the templates when the extension is disabled, we can do it in the extension.

After compilation absolutely all template paths are defined in Twig\Loader\FilesystemLoader::$paths through twig.loader.native_filesystem service, including bundle paths, form theme paths, default project paths, custom project paths, etc.

My point is, shouldn't it be better iterate over the Twig loader paths instead of hardcoding again all possible paths here? (see #27764 where the new default path has been forgotten)

Specifically, this is what I'm talking about:

// ...

$loader = $this->twig->getLoader();
if (!$loader instanceof FilesystemLoader) {
    return;
}

foreach ($loader->getNamespaces() as $namespace) {
    foreach ($loader->getPaths($namespace) as $path) {
        $this->templates = array_merge($this->templates, $this->findTemplatesInDirectory($path, $namespace));
    }
}

// ...

so we don't have to keep this dependency ($paths) updated everywhere:

WDYT?

@yceruto I forgot about this issue, thanks for reminding me. I think refactoring is not subject of this bug issue. You should create a new one.
@yceruto The issue with the form templates is they aren't part of any bundle, they are in bridge package for twig and the iterator doesn't include these paths. The second issue is the templates are part of bridge package for twig and the templates use functions & filters from the form component of symfony. The form component is optional and if the component is not installed then compilation of the templates will fail. The templates should be compiled only in case if the form component is installed & enabled.

I check this bug again and @weaverryan fixed it in the commit 2ef619f, thanks for fix