fix(mapper): materialize promoted defaults for readonly properties

packages/mapper/src/Mappers/ArrayToObjectMapper.php

@@ -64,6 +64,10 @@ public function map(mixed $from, mixed $to): object
             if ($isMissing && ($getHook = $property->getGetHook())) {
                 $from[$propertyName] = $getHook->invokeArgs($targetObject);
             } elseif ($isMissing) {
+                if ($this->applyPromotedDefault($property, $targetObject)) {
+                    continue;
+                }
+
                 $this->handleMissingProperty(
                     property: $property,
                     propertyName: $propertyName,
@@ -133,6 +137,28 @@ private function resolveObject(mixed $objectOrClass): object
         return new ClassReflector($objectOrClass)->newInstanceWithoutConstructor();
     }
 
+    /**
+     * Readonly properties cannot declare a class-level default, so a promoted
+     * constructor parameter is the only place a default can live for them.
+     * Returns false when no default could be applied.
+     */
+    private function applyPromotedDefault(PropertyReflector $property, object $targetObject): bool
+    {
+        if (! $property->isReadonly() || ! $property->hasDefaultValue()) {
+            return false;
+        }
+
+        $parameter = $property->getClass()->getConstructor()?->getParameter($property->getName());
+
+        if ($parameter === null || ! $parameter->isDefaultValueAvailable()) {
+            return false;
+        }
+
+        $property->setValue($targetObject, $parameter->getDefaultValue());
+
+        return true;
+    }
+
     private function setParentRelations(object $parent, ClassReflector $parentClass): void
     {
         static $plans = [];

tests/Integration/Mapper/Fixtures/ObjectWithPromotedDefaults.php

@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Tempest\Integration\Mapper\Fixtures;
+
+final class ObjectWithPromotedDefaults
+{
+    public static int $constructorCalls = 0;
+
+    public function __construct(
+        public readonly string $name = 'default-name',
+        public readonly int $count = 5,
+    ) {
+        self::$constructorCalls++;
+    }
+}

tests/Integration/Mapper/Mappers/ArrayToObjectMapperTest.php

@@ -18,6 +18,7 @@
 use Tests\Tempest\Integration\Mapper\Fixtures\ObjectWithMagicGetter;
 use Tests\Tempest\Integration\Mapper\Fixtures\ObjectWithMyObject;
 use Tests\Tempest\Integration\Mapper\Fixtures\ObjectWithPrimitiveArrayProperties;
+use Tests\Tempest\Integration\Mapper\Fixtures\ObjectWithPromotedDefaults;
 use Tests\Tempest\Integration\Mapper\Fixtures\ParentObject;
 use Tests\Tempest\Integration\Mapper\Fixtures\ParentObjectWithGetterChild;
 use Tests\Tempest\Integration\Mapper\Fixtures\ParentObjectWithVirtualChild;
@@ -72,6 +73,18 @@ public function default_values(): void
         $this->assertNull($object->b);
     }
 
+    #[Test]
+    public function map_promoted_defaults_without_running_constructor(): void
+    {
+        ObjectWithPromotedDefaults::$constructorCalls = 0;
+
+        $object = map(['name' => 'given'])->to(ObjectWithPromotedDefaults::class);
+
+        $this->assertSame('given', $object->name);
+        $this->assertSame(5, $object->count);
+        $this->assertSame(0, ObjectWithPromotedDefaults::$constructorCalls);
+    }
+
     #[Test]
     public function built_in_casters(): void
     {