Can you add a test covering this ?

@stof sure! Just a sec then

That won't work: injecting a string (the REDIS_URL) in RedisAdapter is not allowed.
What you need to do is add a new config option, let's say default_redis_url, and use that to wire the REDIS_URL as a DSN in the same way that I suggested in #20850 (comment) (but programmatically in the DI extension.)

I should review twice before posting :) So this forces any %env(...)% to be understood as a DSN. OK then :)
Does this really work? I mean, env vars are usually seen as unique placeholders from DI ext., not a plain %env()% thing.
Adding a test case will show for sure.

@nicolas-grekas what I am trying to understand is why the service definition you gave in the ticket;

cache.default_redis_provider:
    class: Redis
    public: false
    factory: ["Symfony\\Component\\Cache\\Adapter\\RedisAdapter", "createConnection"]
    arguments:
      - "%env(REDIS_URL)%"

is different from this in PHP;

$definition = new Definition(\Redis::class);
$definition->setPublic(false);
$definition->setFactory(array(RedisAdapter::class, 'createConnection'));
$definition->setArguments(array('%env(REDIS_URL)%'));
$container->setDefinition($name, $definition);

This should do the same in the compilation process right?

@nicolas-grekas ha you typed a response faster than I did. This approach worked in my application so I created this PR to get your opinion before putting any effort in the test cases. The default_redis_provider isn't testing at all btw.

When I apply your patch, I get the same:

[Symfony\Component\DependencyInjection\Exception\EnvParameterException]             
Incompatible use of dynamic environment variables "REDIS_URL" found in parameters.  

The correct patch might be more like:

--- a/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Compiler/CachePoolPass.php
+++ b/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Compiler/CachePoolPass.php
@@ -106,7 +106,9 @@ class CachePoolPass implements CompilerPassInterface
      */
     public static function getServiceProvider(ContainerBuilder $container, $name)
     {
-        if (0 === strpos($name, 'redis://')) {
+        $container->resolveEnvPlaceholders($name, null, $usedEnvs);
+
+        if (0 === strpos($name, 'redis://') || $usedEnvs) {
             $dsn = $name;
 
             if (!$container->hasDefinition($name = md5($dsn))) {

Btw, I'd be OK on my side to have this as bugfix on 3.2.

@nicolas-grekas you are right, I was looking at the wrong thing. I've added the resolve call and updated the testcases to test both the default string and the env var.

👍 (on 3.2 also)

Thank you @robinvdvleuten.

This is the fastest ever submitted+reviewed+fixed+merged+released PR :)

@nicolas-grekas yes indeed! Thanks for all the time and effort you guys put into this process :)

However, it seems that it breaks tests: https://travis-ci.org/symfony/symfony/jobs/183649383

Sorry, this isn't related: #20906

I do think it's related since the test only fails in its redis version with a custom config, it succeeds otherwise