This is BC Break, imho.

Anyone who has implement that interface need update code to change the RTH in implementation of that method.

Thechnically speaking yes, but you need to change only the return type of AuthentictionInterface::authenticate() to IdentityInterface. Your existing code that returns a UserInterface object will remain as is.

This code shows that is possible:

declare(strict_types=1);

interface IdentityInterface {
    public function getIdentity(): string;
}

interface UserInterface extends IdentityInterface {
    public function getRoles() : iterable;
}

interface ClientInterface extends IdentityInterface {
    public function getScopes() : iterable;
}

interface AuthenticationInterface
{
    public function authenticate(string $param): ?IdentityInterface;
}

class Auth implements AuthenticationInterface
{
    public function authenticate(string $param): ?IdentityInterface {
        return new class implements UserInterface {
            public function getIdentity(): string {
                return 'identity';
            }
            public function getRoles() : iterable {
                return [];
            }
        };
    }
}

$a = new Auth(); // no error here

I think this can be an acceptable (small) BC breaks and we can document it in the release note.

@ezimuel

Technically speaking yes, but you need to change only the return type

It is not my decision, but in my opinion we shouldn't release it in minor release. If someone was using it as it was it's going to stop working after running composer update.
For me the question is simply: do we really follow semver or not?

@ezimuel

I think this can be an acceptable (small) BC breaks

"small" or "big" doesn't matter.

@webimpress

For me the question is simply: do we really follow semver or not?

We follow the semantic versioning!

@ezimuel

…we can document it in the release note.

The goal should be: It must work without a user having to read the release notes or documentation. (mini and minor version!)

@froschdesign thanks for explain me how it works the versioning!!!

@weierophinney it's up to you, for me we can release it in a minor/major, no matter. I was just trying to improve the existing code with minimum (one line change code) impact (btw, only if a user has implemented a specific interface). @webimpress and @froschdesign we don't leave in a perfect world.

@froschdesign and @webimpress I removed the "without BC break" in the title and added BC break label.

@ezimuel

thanks for explain me how it works the versioning!!!

I'm sorry, this is and was not my intention. I have only seen the outcry from users which run an update via Composer and did not read the release notes.

Sorry for the misunderstanding!

@froschdesign it's ok. I would have to wait 10 minutes before answering (busy day). I see your point here and I actually agree, this is a BC break (even if it's a really small one).