TLDR: This would be one way to closes #323, and IMHO it's the best one because

• it's correct in 100% of the time for Symfony users and promotes to right correct code.
• People who doesn't care about the Session/SessionInterface can ignore the error.
• People who care about the Session/SessionInterface are not dealing with wrong errors.

This type hint was introduced by
012305d
which was done to replace the original PR
#233 cc @ruudk

As explained in symfony/symfony#39222 (comment)
or symfony/symfony#41765 (comment):

• This is not true that getSession always return a Session
• People should rely on a instanceof checks
  This is even easier now that the FlashBagAwareSessionInterface was introduced in [HttpFoundation] FlashbagAwareSessionInterface symfony/symfony#41765.

It was said by @ruudk

For a small set of users it could mean that phpstan now falsely allows getFlashBag on the request session, because they have a custom session implementation. They prob didn't call the flashbag anyway, as it doesn't work.

But this is not the real problem of the stub.
The real issue is that if you use service decoration, or write correctly your calls to support multiple sessions, like

$session = $request->getSession();
if ($session instanceof FlashBagAwareSessionInterface) {
     $session->getFlashBag()->addMessage('Foo');
} elseif ($session instanceof MyCustomSessionInterface) {
     $session->doThings();
} else {
     // ...
}

You ended up with false positive from phpstan about always true instanceof or always false instanceof.
This is way better to have a "false negative" (`getFlashBag() doesn't exist in SessionInterface) than introducing false positive.

If people doesn't want to write correct code about Symfony session, they should ignore the error

ignoreErros:
     - '#Call to an undefined method Symfony\\Component\\HttpFoundation\\Session\\SessionInterface::getFlashBag\(\)#'

rather than introducing a wrong stub.

Closes #323