bug symfony#25264 [DI] Trigger deprecation when setting a to-be-private synthetic service (nicolas-grekas)

This PR was merged into the 3.4 branch.

Discussion
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[DI] Trigger deprecation when setting a to-be-private synthetic service

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#25253
| License       | MIT
| Doc PR        | -

Commits
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2817576 [DI] Trigger deprecation when setting a to-be-private synthetic service

Author: fabpot

src/Symfony/Component/DependencyInjection/Container.php

@@ -174,7 +174,7 @@ public function set($id, $service)
         }
 
         if (isset($this->privates[$id]) || !(isset($this->fileMap[$id]) || isset($this->methodMap[$id]))) {
-            if (isset($this->syntheticIds[$id]) || (!isset($this->privates[$id]) && !isset($this->getRemovedIds()[$id]))) {
+            if (!isset($this->privates[$id]) && !isset($this->getRemovedIds()[$id])) {
                 // no-op
             } elseif (null === $service) {
                 @trigger_error(sprintf('The "%s" service is private, unsetting it is deprecated since Symfony 3.2 and will fail in 4.0.', $id), E_USER_DEPRECATED);