Narrow return type of `$form->getData()` after calling `$form->isValid()`

Start by reviewing the form getData() and isValid() entry points described in the issue, along with the existing form typing behavior. Determine how a validated form could expose non-null TValidatedData, then add coverage for the before-validation and after-validation types shown in the examples. Done means PHPStan infers the requested types without extra assertions.

Every time I work with forms in Symfony and PHPStan, I'm struggling with making sure PHPStan properly understands what's going on.

After the form has been submitted and validated, I usually end up with doing a lot of assertions to get the typing right.

Let's say we have the following form:

/**
 * @extends AbstractType<array{firstName: string|null, lastName: string|null}>
 */
class UserFormType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options) : void
    {
        $builder->add('firstName', TextType::class, [
            'required' => true,
            'constraints' => [new Length(['min' => 3])],
        ]);
        $builder->add('lastName', TextType::class, [
            'required' => true,
            'constraints' => [new Length(['min' => 3])],
        ]);
    }
}

And we use it like this:

class Controller extends AbstractController
{
    public function addAction(Request $request) : array
    {
        $form = $this->createForm(UserFormType::class);
        $form->handleRequest($request);

        assertType('array{firstName: string|null, lastName: string|null}|null', $form->getData());

        if ($form->isSubmitted() && $form->isValid()) {
            $data = $form->getData();

            assertType('array{firstName: string, lastName: string}', $data);
        }

        return [
            'form' => $form->createView(),
        ];
    }

    public function editAction(Request $request) : array
    {
        $form = $this->createForm(UserFormType::class, [
            'firstName' => 'Ruud',
            'lastName' => 'Kamphuis',
        ]);
        $form->handleRequest($request);

        assertType('array{firstName: string, lastName: string}|null', $form->getData());

        if ($form->isSubmitted() && $form->isValid()) {
            $data = $form->getData();

            assertType('array{firstName: string, lastName: string}', $data);
        }

        return [
            'form' => $form->createView(),
        ];
    }
}

A few things can be improved to make it easier to work with forms.

1. When calling $form->getData() before submitting the form, it returns TData |null. After the form has been submitted and validated, it is still TData|null. At this point I expect it to be TData only.

2. Currently, we can only configure TData on the form. This type should always support the empty states, so this will have a lot of nulls most of the time. What if we introduce a second template TValidatedData, that can returned when calling $form->getData() after the form was submitted and validated?

Would the above be possible? And does it make sense? If so, I could give it a try.