This will be wrong, as some classes may already have been loaded before including bootstrap.php.cache, and this would lead to double declaration of classes

I don't think this argument applies here:ScriptHandler::doBuildBootstrap() is called in a composer plugin context, where the list of currently declared classes has nothing to do with the ones that may be declared whenbootstrap.php.cache will be included by the app.
The only way this works is based on an assumption: that is that thebootstrap.php.cache file is always included first or that the classes it declares won't collide with maybe preloaded classes. Not having this assumption in place would mean that things are broken by design.
A pragmatic argument that support this assumption is that SensioDistributionBundle is part of Symfony Standard Edtion and is by no mean meant for extensibility. That's whybootstrap.php.cache is and can be included first and unconditionally in e.g.app.php.

I forgot to write it but we should also consider that the list of classes we're talking about is hard coded. Thus, any of the listed classes were intended for inlining.

does it actually change anything for bootstrap.php.cache ? This file is not created in an adaptive way, and so is not impacted by declared classes

One class is re-inlined by this fix currently: ClassCollectionLoader itself.
Declared classes affect the result no matter what$adaptive is, seesymfony/symfony@a1031f6#diff-bc09f5d8b58e88e3a673386fdee7a981

this looks weird to me. It means that the file is always adaptive.

It is, but bootstrapping is a complex topic :)

Updated: the list of classes is too large. I reduced it to classes that are needed before kernel booting.
All removed classes here are now listed in FrameworkExtension insymfony/symfony#19276

"The list of classes is too large" -> what's the consequence?

This is a matter of responsibility, to where this stops and delegates to the "addClassesToCompile" mechanism. Practically, all classes listed here should be listed in linked PR for exclusion as well. The shorter the list, the easier the sync in the long run.

ping@fabpot , this one needs to be merged to make the standard edition work

Is it BC with older versions of Symfony?

Yes it should: less classes inlined. And the method_exists check.

Just confirmed with a symfony-standard 2.7: runs just fine with this patch.

ping@fabpot

Any blocker here?

Thank you@nicolas-grekas