The only solution I found for this is explained in this blog post - https://www.computerminds.co.uk/articles/apply-drupal-9-compatibility-pa... - which basically boils down to:
- telling composer to use the source git repo for the package instead of using the official Drupal 'packages' repo
"repositories": [
{
"type": "package",
"package": {
"name": "drupal_git/belgrade",
"type": "drupal-module",
"version": "dev-8.x-1.x",
"source": {
"type": "git",
"url": "https://git.drupalcode.org/project/belgrade.git",
"reference": "efdc324ae8c48fca9e6fce81781eece0cee0c313"
}
}
},
{
"type": "composer",
"url": "https://packages.drupal.org/8"
}
],- (making sure you require the cweagans/composer-patches package)
- applying the patch in the normal way in composer.json inside "extra"
"patches": {
"drupal/belgrade": {
"Drupal 9.0 support": "https://www.drupal.org/files/issues/2020-11-15/d9-ready-3151610-3.patch"
}
},- then requiring the project - I did this yesterday after trying several things so I can't remember exactly whether it worked with composer require or composer update
Unfortunately I tried so many things before I got it working that I'm not yet entirely sure whether it worked due to the fact I added the bit mentioned in one of the comments in the blog post above, which is basically faking drupal 9 to be 'seen' as drupal 8:
"drupal/core": "9.1.4 as 8.9.99",
So if you would still have an issue after the points above then can always try this.
But anyway, back to the point, the patch works great and the theme installs and works great after this one line patch so I'd say the quicker it gets committed the less people would have to struggle requiring it on a new D9 site.