Symfony version(s) affected: 3.4.0 and higher

Description
There is getBag method at Symfony\Component\HttpFoundation\Session which calls $this->storage->getBag($name)->getBag().

As defined in constructor $this->storage is instance of Symfony\Component\HttpFoundation\Session\Storage\SessionStorageInterface which contains method getBag(string $name). As defined in comment it is expected that SessionStorageInterface::getBag(string $name) should return instance of Symfony\Component\HttpFoundation\Session\SessionBagInterface.

The problem is that SessionBagInterface does not contain getBag method so it's impossible to expect that $this->storage->getBag($name) will return object containing getBag() method.

How to reproduce

use Symfony\Component\HttpFoundation\Session\Attribute\AttributeBag;
use Symfony\Component\HttpFoundation\Session\Session;
use Symfony\Component\HttpFoundation\Session\Storage\MockArraySessionStorage;

$bag = new AtrributeBag();
$bag->setName('foo');

$storage = new MockArraySessionStorage();
$storage->registerBag($bag);

$session = new Session($storage);
$session->getBag('foo'); // this will effect with "Error: Call to undefined method Symfony\Component\HttpFoundation\Session\Attribute\AttributeBag::getBag()

Possible Solution
I'm not sure, probably removing second getBag call from Session::getBag($name).