There are related failures in the test suite, I'm looking into it.

So I've dug a little more to properly implement these improvements. Hopefully, I got a good enough understanding now.

Mainly I've been reminded that

$this->foo(...) === $this->foo(...) // false
[$this, 'foo'] === [$this, 'foo'] // true

Even though both are callables, they are Closure and array respectively. So switching one with the other is not as straightforward as I previously thought.

This has consequences on unit tests where equality is tested. Symfony public api returning callables with the array syntax form must be kept that way and therefore their related tests too.

A couple more points:

• As stated by @stof \Symfony\Component\DependencyInjection\Definition::setFactory doesn't support closures.

• I refrained from applying this in \Symfony\Component\EventDispatcher\EventDispatcher::addSubscriber

--- a/src/Symfony/Component/EventDispatcher/EventDispatcher.php
+++ b/src/Symfony/Component/EventDispatcher/EventDispatcher.php
@@ -163,10 +163,10 @@ class EventDispatcher implements EventDispatcherInterface
             if (\is_string($params)) {
                 $this->addListener($eventName, [$subscriber, $params]);
             } elseif (\is_string($params[0])) {
-                $this->addListener($eventName, [$subscriber, $params[0]], $params[1] ?? 0);
+                $this->addListener($eventName, $subscriber->{$params[0]}(...), $params[1] ?? 0);
             } else {
                 foreach ($params as $listener) {
-                    $this->addListener($eventName, [$subscriber, $listener[0]], $listener[1] ?? 0);
+                    $this->addListener($eventName, $subscriber->{$params[0]}(...), $listener[1] ?? 0);
                 }
             }
         }

As it changes the expected behavior of what's stored inside the listeners (closure instead of array) when adding a Subscriber.
Even though it's valid, as the param is typed array|callable, it could break userland code if they implicitly rely on having an array here when addSubscriber is called.

• I've reverted updates on EventDispatcherTest::testGetLazyListeners , testRemoveFindsLazyListeners, testPriorityFindsLazyListeners as it fails and I don't understand what it's actually testing to fix them. Maybe @nicolas-grekas can provide some guidance here 🙏
• I've reverted updates on the ErrorHandler component as it breaks a lot of tests and there's too much dark magic going on to make the modernizing effort worth it.
• It seems \Symfony\Component\HttpKernel\Controller\ArgumentResolver\ServiceValueResolver::resolve doesn't support $request->attributes->get('_controller') returning a Closure. TBC.

Ready to be reviewed again. Test failures are not related.

@nicolas-grekas Thanks for your thorough review, very motivating :)

Ready to be reviewed again. 👍

Thank you @tigitz.