I added a commit to fix the psalm build @stof ; but I don't know why it wasn't failing before.

I don't understand this change. The current annotations look correct to me and why would FormErrorIterator need a type variable?

I don't understand this change. The current annotations look correct to me and why would FormErrorIterator need a type variable?

Currently $form->getErrors() is returning FormErrorIterator which means a FormErrorIterator<FormError|FormErrorIterator>.

This can be more precise according to
https://github.com/symfony/symfony/blob/6.1/src/Symfony/Component/Form/FormInterface.php#L90-L94

$form->getErrors(), $form->getErrors(false), $form->getErrors(true) or $form->getErrors(true, true) are only returning a flatten result. So it's a FormErrorIterator<FormError> which is more precise than FormErrorIterator<FormError|FormErrorIterator>. This allow to avoid useless checks when iterating on the errors.

ReturnTypeProvider for phpstan/psalm can be implemented and already half done but they require the class to be a generic.

Knowing the result is only a FormErrorIterator can allow to write

        $errors = [];
        foreach ($form->getErrors(true) as $error) {
            $errors[] = $error->getMessage();
        }

without any complaints.

$form->getErrors(), $form->getErrors(false), $form->getErrors(true) or $form->getErrors(true, true) are only returning a flatten result.

Oh right. Makes sense, thanks for the explanation.

Thank you @VincentLanglet.