Hello @crtl, sorry for the late response.

In fact your example in not exactly the same but quite similar to the one of your first link.
What is missing is what's documented in the Collection constraint which considers all keys as required by default in all group, so your example is configured as if it was the following:

$constraints = new Assert\Collection([
    "fields" => [
        "type" => new Assert\Required([
            "constraints" => [
                new Assert\Choice([
                    "choices" => ["credentials", "google"],
                    "groups" => ["type"]
                ]),
            ],
            "groups" => ["type", "google"],
        ]),
        "token" => new Assert\Required([
            "constraints" => [
                new Assert\NotBlank([
                    "groups" => ["google"],
                ],
            ],
            "groups" => ["type", "google"],
        ]),
    ],
    // the collection constraint must be traversed for each nested group
    "groups" => ["type", "google"],
    "allowMissingFields" => false,
]);

So each field is required in all groups by default, you need to change this one line in your example to get the intended behavior:

-"token" => new Assert\NotBlank([
+"token" => new Assert\Optional(new Assert\NotBlank([
    "groups" => ["google"]
-])
+]))

@symfony/team-symfony-docs do you think we need a PR for this? Thanks!

I want really aware of this Required and Optional stuff. I DO think we should document this better :)

Thanks for the input @weaverryan, then I suggest to update this article https://symfony.com/doc/current/validation/raw_values.html to add a new section with groups, to tell about what I've explained in my comment above. WDYT?

That looks right. But we should also link to that (if we don’t already) from the CollectionType

I guess you're thinking about the Collection constraint, as I was thinking about it too :).

In fact it fitted more smoothly in the Collection constraint doc itself with a proper note from the article (the opposite of my first thought), I've opened #13492.