bug #31418 [FrameworkBundle] clarify the possible class/interface of …

…the cache (xabbuh)

This PR was merged into the 3.4 branch.

Discussion
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[FrameworkBundle] clarify the possible class/interface of the cache

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

When the fallback cache pool is returned (on PHP 5.6, HHVM, or when
Opcache is disabled), the configured service can be any implementation
of the CacheItemPoolInterface.

Commits
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4027374 clarify the possible class/interface of the cache

src/Symfony/Bundle/FrameworkBundle/Resources/config/serializer.xml

@@ -82,7 +82,7 @@
             <tag name="kernel.cache_warmer" />
         </service>
 
-        <service id="serializer.mapping.cache.symfony" class="Symfony\Component\Cache\Adapter\PhpArrayAdapter">
+        <service id="serializer.mapping.cache.symfony" class="Psr\Cache\CacheItemPoolInterface">
             <factory class="Symfony\Component\Cache\Adapter\PhpArrayAdapter" method="create" />
             <argument>%serializer.mapping.cache.file%</argument>
             <argument type="service" id="cache.serializer" />