Description
The current SessionInterface does not contain the method getFlashBag. As the DI way has been deprecated it's difficult to get correctly the FlashBag.

The workaround currently is

// Returns SessionInterface without getFlashBag
$session = $request->getSession();

if (method_exists($session, 'getFlashBag') {
$session->getFlashBag()->fooo();
}

or asserting that is a actual Session object which makes decoration shitty

Example

Introduce an additional interface to access the method getFlashBag on the interface level and not implementation.