What issue does this fix considering we already have a dedicated contraint?

Currently you have to do everything manually:

new Callback(static function ($collection, ExecutionContextInterface $context): void {
    $context
        ->getValidator()
        ->inContext($context)
        ->validate(\count($collection), new DivisibleBy([
            'value' => 3,
        ]));
}),

What about supporting countable in https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Validator/Constraints/DivisibleByValidator.php#L21 this instead?

I thought about this but Count is made for collections. Otherwise, we should also support countable for other comparisons constraints such as GreaterThan, LessThan, etc.

Why not? :)

The collection « count » is divisible by X, not the collection itself.

It is usually different, but when an object is countable, this is just the same thing.

I think this approach (this PR) is more clear to me also.

Thank you @fancyweb.