Deprecate SequenceGenerator implementing Serializable

UPGRADE.md

@@ -1,5 +1,15 @@
 # Upgrade to 3.2
 
+## Deprecate remaining `Serializable` implementation
+
+Relying on `SequenceGenerator` implementing the `Serializable` is deprecated
+because that interface won't be implemented in ORM 4 anymore.
+
+The following methods are deprecated:
+
+* `SequenceGenerator::serialize()`
+* `SequenceGenerator::unserialize()`
+
 ## `orm:schema-tool:update` option `--complete` is deprecated
 
 That option behaves as a no-op, and is deprecated. It will be removed in 4.0.

src/Id/SequenceGenerator.php

@@ -5,6 +5,7 @@
 namespace Doctrine\ORM\Id;
 
 use Doctrine\DBAL\Connections\PrimaryReadReplicaConnection;
+use Doctrine\Deprecations\Deprecation;
 use Doctrine\ORM\EntityManagerInterface;
 use Serializable;
 
@@ -65,8 +66,17 @@ public function getNextValue(): int
         return $this->nextValue;
     }
 
+    /** @deprecated without replacement. */
     final public function serialize(): string
     {
+        Deprecation::trigger(
+            'doctrine/orm',
+            'https://github.com/doctrine/orm/pull/11468',
+            '%s() is deprecated, use __serialize() instead. %s won\'t implement the Serializable interface anymore in ORM 4.',
+            __METHOD__,
+            self::class,
+        );
+
         return serialize($this->__serialize());
     }
 
@@ -79,8 +89,17 @@ public function __serialize(): array
         ];
     }
 
+    /** @deprecated without replacement. */
     final public function unserialize(string $serialized): void
     {
+        Deprecation::trigger(
+            'doctrine/orm',
+            'https://github.com/doctrine/orm/pull/11468',
+            '%s() is deprecated, use __unserialize() instead. %s won\'t implement the Serializable interface anymore in ORM 4.',
+            __METHOD__,
+            self::class,
+        );
+
         $this->__unserialize(unserialize($serialized));
     }