Question about generics and required verbosity

[petervossen]

Playground: https://phpstan.org/r/e0ebd5cf-8308-44fe-98a8-5047f48fc3f1

Method getAWithError gives the 'should return TA of A but returns A' error. I'm trying to understand why the @var in getAWithoutError is necessary. Can someone explain to me what I am missing here?

/** @return TA */
public function getAWithError() : A { 
    return $this->b->a; 
}
/** @return TA */
public function getAWithoutError() : A { 
    /** @var TA $ta */
    $ta = $this->b->a; 
    return $ta;
}

[ondrejmirtes]

The tip on the playground tries to explain it:

Type A is not always the same as TA. It breaks the contract for some argument types, typically subtypes.

What it means: Let's say you have C<X, Y>. This means that getAWithError has to return X.

If you have C<Alpha, Beta>, getAWithError needs to return Alpha.

Your types do not guarantee that. This is the fix: https://phpstan.org/r/72a9e0a7-4c7d-4ed7-9977-64b1df59ca05

[petervossen]

That makes sense, thank you.