fix: detect invalid constructor handle type

An exception will now be thrown when a constructor is registered for
an invalid type: something other than a class.

src/Mapper/Object/Exception/InvalidClassConstructorType.php

@@ -0,0 +1,21 @@
+<?php
+
+declare(strict_types=1);
+
+namespace CuyZ\Valinor\Mapper\Object\Exception;
+
+use CuyZ\Valinor\Definition\FunctionDefinition;
+use CuyZ\Valinor\Type\Type;
+use RuntimeException;
+
+/** @internal */
+final class InvalidClassConstructorType extends RuntimeException
+{
+    public function __construct(FunctionDefinition $function, Type $type)
+    {
+        parent::__construct(
+            "Invalid type `{$type->toString()}` handled by constructor `{$function->signature()}`. It must be a valid class name.",
+            1659446121
+        );
+    }
+}

src/Mapper/Object/Factory/ConstructorObjectBuilderFactory.php

@@ -7,9 +7,11 @@
 use CuyZ\Valinor\Definition\ClassDefinition;
 use CuyZ\Valinor\Definition\FunctionsContainer;
 use CuyZ\Valinor\Mapper\Object\Exception\CannotInstantiateObject;
+use CuyZ\Valinor\Mapper\Object\Exception\InvalidClassConstructorType;
 use CuyZ\Valinor\Mapper\Object\FunctionObjectBuilder;
 use CuyZ\Valinor\Mapper\Object\MethodObjectBuilder;
 use CuyZ\Valinor\Mapper\Object\ObjectBuilder;
+use CuyZ\Valinor\Type\Types\ClassType;
 
 use function array_key_exists;
 use function array_unshift;
@@ -70,7 +72,13 @@ private function listBuilders(ClassDefinition $class): array
             $methods = $class->methods();
 
             foreach ($this->constructors as $constructor) {
-                if ($constructor->definition()->returnType()->matches($type)) {
+                $handledType = $constructor->definition()->returnType();
+
+                if (! $handledType instanceof ClassType) {
+                    throw new InvalidClassConstructorType($constructor->definition(), $handledType);
+                }
+
+                if ($handledType->matches($type)) {
                     $builders[] = new FunctionObjectBuilder($constructor);
                 }
             }

tests/Integration/Mapping/ConstructorRegistrationMappingTest.php

@@ -6,6 +6,7 @@
 
 use CuyZ\Valinor\Mapper\MappingError;
 use CuyZ\Valinor\Mapper\Object\Exception\CannotInstantiateObject;
+use CuyZ\Valinor\Mapper\Object\Exception\InvalidClassConstructorType;
 use CuyZ\Valinor\Mapper\Object\Exception\ObjectBuildersCollision;
 use CuyZ\Valinor\MapperBuilder;
 use CuyZ\Valinor\Tests\Fake\Mapper\Tree\Message\FakeErrorMessage;
@@ -387,6 +388,18 @@ public function test_non_registered_named_constructors_are_ignored(): void
             ->map(SomeClassWithPrivateNativeConstructor::class, []);
     }
 
+    public function test_invalid_constructor_type_throws_exception(): void
+    {
+        $this->expectException(InvalidClassConstructorType::class);
+        $this->expectExceptionCode(1659446121);
+        $this->expectExceptionMessageMatches('/Invalid type `string` handled by constructor `.*`\. It must be a valid class name\./');
+
+        (new MapperBuilder())
+            ->registerConstructor(fn (): string => 'foo')
+            ->mapper()
+            ->map(stdClass::class, []);
+    }
+
     public function test_registered_datetime_constructor_is_used(): void
     {
         $default = new DateTime('@1356097062');