Can confirm the issue.

There is a "namespace" forced as the first argument here:

https://github.com/symfony/symfony/blob/master/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Compiler/CachePoolPass.php#L84

Seems ArrayAdapter is excluded there already... probably then all adapters should be excluded where the first argument of the constructor is not a namespace? ping @nicolas-grekas

I suppose the current doc could be improved. Would you mind opening a PR doing so?
Here are the explanations: the framework.cache.* entries are here to define new cache pools without doing direct service configuration. Mixing both styles requires knowing how config creates services.
So, what do you want to do?
There are two errors in the example you give:

• the chain takes concrete services as arguments, but cache.adapter.* are abstract ones.
• the framework.cache.app config defines a new pool bound to a service named cache.app.

So, if you want to override the cache.app service, just do it by naming your chain "cache.app" (or by creating it as an alias, both should work.)
For the 1st issue, you can create pools using the config, then reference them in your chain service definition.

Thank you for the insight. I'll be happy to open a PR to the ChainAdapter docs. I'd appreciate some additional clarification first though.

I got it working with the config below. I believe that's what you suggested in your comment. However I'm wondering if it can be simplified. Can the chain adapter be configured in framework.cache without adding a service definition? Since the only thing that service is doing is passing the apcu and redis adapters as arguments.

# framework.cache
pools:
    app.cache.apcu:
        adapter: cache.adapter.apcu
        default_lifetime: 86400
    app.cache.redis:
        adapter: cache.adapter.redis
        default_lifetime: 86400
# services.yaml
    cache.app:
        class: Symfony\Component\Cache\Adapter\ChainAdapter
        arguments:
            $adapters: ['@app.cache.apcu', '@app.cache.redis']

Yes, that's exactly what I suggested.

the only thing that service is doing is passing the apcu and redis adapters as arguments

You're missing the name of the pools, from which a namespace is derivated.

I tried this config, without the service

        pools:
            app.cache.apcu:
                name: cache.app.apcu
                adapter: cache.adapter.apcu
                default_lifetime: 86400
            app.cache.redis:
                name: cache.app.redis
                adapter: cache.adapter.redis
                default_lifetime: 86400

However, when I debug the container I get two different TraceableAdapter services. e.g.

Information for Service "cache.app.apcu"
========================================
  Service ID       cache.app.apcu
  Class            Symfony\Component\Cache\Adapter\TraceableAdapter

When I manually defined the service overwriting cache.app, I was getting the expected ChainAdapter

Information for Service "cache.app"
===================================
  Service ID       cache.app
  Class            Symfony\Component\Cache\Adapter\ChainAdapter

I'm not sure if I'm missing something in my pools config. I tried using the same name for both hoping that the two adapters in the same namespace would be put together in a ChainAdapter but instead I get a DuplicateKeyException.

Real pool are wrapped in a TraceableAdapter to make the profiler panel work for them. All is fine :)