Declare type for Exception::$message and Error::$message

[kocsismate]

Although I proposed not to declare these property types in #6891 (comment) with respect to BC, I realized afterwards that both the related constructor parameter and the getter's return type is declared as string since PHP 8.0, so I'm wondering if we could yet add these types?

[nikic]

See https://github.com/symfony/symfony/blob/bb1e1e58aea5318e96d1c22cc8a91668ed7baaaa/src/Symfony/Component/DependencyInjection/Exception/AutowiringFailedException.php#L41 for a usage. It assigns the property directly and doesn't go through the constructor.

[nikic]

I think it would be good to add a test for this kind of usage, so we don't break it in the future.

[kocsismate]

Ah, thank you for the example. I'm closing the PR then :) If I'll have time, I'll try to add a test for this!