I wonder what prevents us from allowing HttpKernel ^4.4 to be used with ErrorHandler ^5.0?

This was done in7ec445b by@nicolas-grekas. Not sure if he still knows why he did that back then.

Thank you Matthias.

Your pace is awesome, thank you guys!

⚠️ see comment atsymfony/http-kernel@486baf0.

Cannot tell right now if that’s an incompatibility between Symfony packages and/or the person did not declare their dependencies correctly.

Well, they probably don't have a direct dependency on symfony/error-handler in their project, and upgrading to version 5 of the error handler removed the BC layer with symfony/debug (but for a project upgrading Symfony 4 from minor to minor, not having a requirement on symfony/error-handler is quite expected as earlier versions of symfony/http-kernel were using symfony/debug for that feature)

I think the situation is as follows:

• The ExceptionController in TwigBundle 4.4 expects a Symfony\Component\Debug\Exception\FlattenException (here)
• In ErrorHandler 4.4, the Symfony\Component\ErrorHandler\Exception\FlattenException isa subclass of the (at that time already deprecated) Symfony\Component\Debug\Exception\FlattenException
• With the change from this PR, ErrorHandler 5.x may be installed, where Symfony\Component\ErrorHandler\Exception\FlattenException no longer extends the legacy base class (here)

I might be wrong, but is this a missing depencendy declaration (either requires or maybe conflicts) in TwigBundle 4.4 against either ErrorHandler or the Debug component?

Reasons against this: TwigBundle does not require either one, and also has no problem when ErrorHandler 5.x is installed... it just can't handle exceptions generated in ErrorHandler 5.x.

Another suggestion: I could change the ExceptionController in TwigBundle 4.4 to be kind of forwards-compatible and accept either Exceptions from Debugor ErrorHandler. But that change would probably be breaking BC for users inheriting from that Controller and overwriting the method, right?

Would it be OK if ErrorHandler'sFlattenException in 5.4 would still inherit from Symfony\Component\Debug\Exception\FlattenException if that class exists (decided at runtime)?

The point of doing a major version bump is to get rid of those hacks. I think we should revert this PR. That's quite unfortunate but we cannot afford breaking existing stacks I fear.

I agree about reverting that PR.

Also agree!

Should we learn from this that some kind of test is missing… if so, what might that be?

The missing test would be a functional test triggering the rendering of an error page.

Reverted in#46365