Correct: we removed the dependency from symfony/cache so you need to declare it explicitly in your app now. Maybe worth a CHANGELOG/UPGRADE entry? Would you mind sending a PR doing so?

What troubles me a bit is that Symfony 4.2 applications that don't use the PSR interface directly suddenly fail after the upgrade, for instance when instantiating (the now deprecated) Symfony\Component\Cache\Simple\FilesystemCache.

My application used Simple Cache, which is now deprecated.
I corrected it, according to Cache Contracts.

I think that while Simple Cache exists, psr/simple-cache should exist in dependencies.

I think that while Simple Cache exists, psr/simple-cache should exist in dependencies.

Either that or we need to make sure that the deprecated classes remain usable without the PSR interface.

Oh, it's already in the changelog and upgrade files, all good on my side, there is no other way I know about.

make sure that the deprecated classes remain usable without the PSR interface

that's not possible, as the very purpose of the classes in the Simple namespace is to pass the psr/simple-cache type hints.

the very purpose of the classes in the Simple namespace is to pass the psr/simple-cache type hints.

Fair enough. In the project where I fixed this issue a couple of days ago, the developers just used FilesystemCache standalone because it's a cache that did what they needed it to do. They didn't really care about the fact that it implemented the PSR interface.

I might have missed the discussion on the topic, but would it be that problematic to keep the dependency for now and remove it together with the deprecated implementations in 5.0? I get the impression that the upgrade path might be smoother that way since developers would get a proper deprecation notice instead of a fatal error.

That'd smoothen for ppl that move to 4.3 and do the change before 4.4. Thus it wouldn't change the core process. But we can yes.

Closing as "not a bug". We can smoothen, but the more we wait the less an issue this is :)
This also matches our policy regarding dependencies.

@nicolas-grekas isn't that a breaking change though? We are upgrading from the previous minor, and our apps are breaking due to this change...

Mysterious failure on AppVeyor: https://ci.appveyor.com/project/dunglas/dunglasapibundle/builds/25778904#L340

Something different about Windows, I suppose.

@teohhanhui Could be a variant of #32395

Curiously enough, it doesn't happen on master branch of API Platform, but only 2.4. Must be due to a different combination of package versions...

And of course, it only happens on Windows in our case. Lol...

TL;DR: composer req psr/simple-cache