When I try to call ResolvedFormType::getTypeExtensions() I get an error:

Attempted to call method "getExtensions" on class "Symfony\Component\Form\Extension\Core\Type\FormType".

As I see there are code for BC ( https://github.com/symfony/Form/blob/master/ResolvedFormType.php#L98 ):

    public function getTypeExtensions()
    {
        // BC
        if ($this->innerType instanceof AbstractType) {
            return $this->innerType->getExtensions();
        }

        return $this->typeExtensions;
    }

but we don't have use statement for deprecated AbstractType:

use Symfony\Component\Form\Exception\InvalidArgumentException;
use Symfony\Component\Form\Exception\UnexpectedTypeException;
use Symfony\Component\EventDispatcher\EventDispatcher;
use Symfony\Component\OptionsResolver\OptionsResolver;

so it is assumed that deprecated AbstractType class is stored in the same folder as ResolvedFormType class. But current AbstractType is also stored in the same folder with ResolvedFormType and it doesn't have getExtensions() method: https://github.com/symfony/Form/blob/master/AbstractType.php