fix(serializer): forward DiscriminatorMap defaultType in PropertyMetadataLoader

src/Serializer/Mapping/Loader/PropertyMetadataLoader.php

@@ -71,7 +71,8 @@ public function loadClassMetadata(ClassMetadataInterface $classMetadata): bool
             if ($attribute instanceof DiscriminatorMap) {
                 $classMetadata->setClassDiscriminatorMapping(new ClassDiscriminatorMapping(
                     method_exists($attribute, 'getTypeProperty') ? $attribute->getTypeProperty() : $attribute->typeProperty,
-                    method_exists($attribute, 'getMapping') ? $attribute->getMapping() : $attribute->mapping
+                    method_exists($attribute, 'getMapping') ? $attribute->getMapping() : $attribute->mapping,
+                    method_exists($attribute, 'getDefaultType') ? $attribute->getDefaultType() : ($attribute->defaultType ?? null),
                 ));
                 continue;
             }

src/Serializer/Tests/Fixtures/Model/AbstractWithDiscriminator.php

@@ -0,0 +1,21 @@
+<?php
+
+/*
+ * This file is part of the API Platform project.
+ *
+ * (c) Kévin Dunglas <dunglas@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+namespace ApiPlatform\Serializer\Tests\Fixtures\Model;
+
+use Symfony\Component\Serializer\Attribute\DiscriminatorMap;
+
+#[DiscriminatorMap(typeProperty: 'discr', mapping: ['concrete' => ConcreteWithDiscriminator::class], defaultType: 'concrete')]
+abstract class AbstractWithDiscriminator
+{
+}

src/Serializer/Tests/Fixtures/Model/ConcreteWithDiscriminator.php

@@ -0,0 +1,18 @@
+<?php
+
+/*
+ * This file is part of the API Platform project.
+ *
+ * (c) Kévin Dunglas <dunglas@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+namespace ApiPlatform\Serializer\Tests\Fixtures\Model;
+
+class ConcreteWithDiscriminator extends AbstractWithDiscriminator
+{
+}

src/Serializer/Tests/Mapping/Loader/PropertyMetadataLoaderTest.php

@@ -16,9 +16,11 @@
 use ApiPlatform\Metadata\Property\Factory\PropertyNameCollectionFactoryInterface;
 use ApiPlatform\Metadata\Property\PropertyNameCollection;
 use ApiPlatform\Serializer\Mapping\Loader\PropertyMetadataLoader;
+use ApiPlatform\Serializer\Tests\Fixtures\Model\AbstractWithDiscriminator;
 use ApiPlatform\Serializer\Tests\Fixtures\Model\HasRelation;
 use ApiPlatform\Serializer\Tests\Fixtures\Model\Relation;
 use PHPUnit\Framework\TestCase;
+use Symfony\Component\Serializer\Mapping\ClassDiscriminatorMapping;
 use Symfony\Component\Serializer\Mapping\ClassMetadata;
 
 final class PropertyMetadataLoaderTest extends TestCase
@@ -55,4 +57,21 @@ public function testCreateMappingForAClass(): void
         $this->assertArrayHasKey('name', $attributesMetadata);
         $this->assertEquals(['read'], $attributesMetadata['name']->getGroups());
     }
+
+    public function testForwardsDiscriminatorDefaultType(): void
+    {
+        if (!method_exists(ClassDiscriminatorMapping::class, 'getDefaultType')) { // @phpstan-ignore-line
+            $this->markTestSkipped('ClassDiscriminatorMapping::getDefaultType() requires symfony/serializer 7.1+.');
+        }
+
+        $coll = $this->createStub(PropertyNameCollectionFactoryInterface::class);
+        $coll->method('create')->willReturn(new PropertyNameCollection([]));
+        $loader = new PropertyMetadataLoader($coll);
+        $classMetadata = new ClassMetadata(AbstractWithDiscriminator::class);
+        $loader->loadClassMetadata($classMetadata);
+
+        $mapping = $classMetadata->getClassDiscriminatorMapping();
+        $this->assertNotNull($mapping);
+        $this->assertSame('concrete', $mapping->getDefaultType());
+    }
 }