Well, you will need to register the DoctrineExtension in your TypeTestCase if you rely on the Doctrine types in the form you test.
The TypeTestCase is not using the container (it does not even boot the kernel and does not even require having a kernel) to register form types.

Thank you, how would you do this ? And should it be added to the documentation ?

See the next chapter in the docs you linked: https://symfony.com/doc/current/form/unit_testing.html#adding-custom-extensions

See the next chapter in the docs you linked: https://symfony.com/doc/current/form/unit_testing.html#adding-custom-extensions

IMHO, this is not clear enough.

First, without documentation, I don't think everybody will know that the DoctrineOrmExtension is required for a form with a EntityType.

Then, the extension is not easy to use. You have to write

protected function getExtensions(): array
    {
        return [
            new DoctrineOrmExtension($registry),
        ];
    }

But you need a $registry !

If we go deep in some tests written by Symfony, we can find a way to get a $registry:

$entityManager = DoctrineTestHelper::createTestEntityManager();
$registry = $this->createMock(ManagerRegistry::class);

$registry->method('getManager')->willReturn($entityManager);
$registry->method('getManagerForClass')->willReturn($entityManager);

return $registry;

This can also be found in StackOverflow

But this only works with EntityRepository, not ServiceEntityRepository !
With a ServiceEntityRepository, you receive the following error message

TypeError: Argument 1 passed to FooRepository::__construct() must implement interface Doctrine\Persistence\ManagerRegistry, instance of Doctrine\ORM\EntityManager given

This error can be found on StackOverflow without any resolution.

Maybe I missed an easy solution but in my case, I only made it work with:

$factory = new TestRepositoryFactory();
$configuration->setRepositoryFactory($factory);
$entityManager = DoctrineTestHelper::createTestEntityManager($configuration);

$repository = $this->createMock(EntityRepository::class);
$repository->method('createQueryBuilder')->willReturn($this->createMock(QueryBuilder::class));
$factory->setRepository($entityManager, Foo::class, $repository);

Which is far from being easy.

Hello @VincentLanglet, I agree we need a doc update, I have opened symfony/symfony-docs#13552.

The KernelTestCase (instead of the TypeTestCase) should be your way to go in this case, using the "real" form factory.

I think we should close here though as there is nothing we can do about it in core.

The KernelTestCase (instead of the TypeTestCase) should be your way to go in this case, using the "real" form factory.

Indeed, the kernel would be useful.
But the TypeTestCase would still be necessary if I want to add form extensions and stuff, isn't it ?
Should the TypeTestCase extend the KernelTestCase then ?

But the TypeTestCase would still be necessary if I want to add form extensions and stuff, isn't it ?
Should the TypeTestCase extend the KernelTestCase then ?

Both should not be related. The KernelTestCase is using the "real" container (based on test env config though) so you don't have to do anything more since the form factory will be loaded as a service and will register everything just like when your application is handling a request.

Closing as that's related to the doc and this staled here.