I understand covariance and contravariance in PHP classes/methods/functions. I apparently do not understand it in generics.

#1

I have a class OrderUpdater that works for any model. It takes a ModelCollection of any model, and updates the models within. Constructor bit:

class OrderUpdater {

	/** @var ModelCollection<int, Model> */
	protected ModelCollection $models;

	/** @param ModelCollection<int, Model> $models */
	public function __construct(ModelCollection $models) {
		$this->models = $models->keyBy('id');
	}

}

(The smart people probably immediately see the problem with ModelCollection<int, Model> because it never takes literal Model objects.)

My controller makes an OrderUpdater:

$tags = $organization->tags;
\PHPStan\dumpType($tags); // App\Base\ModelCollection<int, App\Models\Tag>

(new OrderUpdater($tags))->updateWithTransaction((array) $request['o']);

And then PhpStan complains:

Parameter #1 $models of class App\Base\OrderUpdater constructor expects App\Base\ModelCollection<int, App\Base\Model>, App\Base\ModelCollection<int, App\Models\Tag> given.

• 💡 Template type TModel on class App\Base\ModelCollection is not covariant. Learn more:
• https://phpstan.org/blog/whats-up-with-template-covariant

I've read that page 6 times now, and I definitely don't get it. Why doesn't it accept a Tag if it needs a Model?? (That's probably not what it means, but that's what it looks like to me.)

#2

So I thought, screw it, I'll just make ModelCollection's TModel covariant:

/**
 * @template TKey of array-key
 * @template-covariant TModel of Model
 *
 * @extends EloquentCollection<TKey, TModel>
 */
class ModelCollection extends EloquentCollection {

but now PhpStan complains about that covariant template!

Template type TModel is declared as covariant, but occurs in invariant position in extended type Illuminate\Database\Eloquent\Collection<TKey of (int|string), TModel of App\Base\Model> of class App\Base\ModelCollection.

So that's also not how that works.

#3

I found out ModelCollection<int, *> is a thing earlier, so that's also an option I thought. Instead of OrderUpdater requiring Model it could require *:

class OrderUpdater {

	/** @var ModelCollection<int, *> */
	protected ModelCollection $models;

	/** @param ModelCollection<int, *> $models */
	public function __construct(ModelCollection $models) {
		$this->models = $models->keyBy('id');
	}

}

and then everything works for creating the OrderUpdater object, BUT not when the class actually acts on the object:

$this->models[$id]->update(['o' => $o * 10]);

because apparently * doesn't mean 'any Model':

Cannot call method update() on mixed.

but mixed?? Really?? Then what is the point of *? Of course it's a Model, that's the requirement in the @template!

I'm losing my mind. Please save me, explain to me in little baby language what generic covariance is, and how I can make my ModelCollection AND OrderUpdater work.