Do not fatal when a shorthand service class cannot be loaded

src/Drupal/DrupalAutoloader.php

@@ -237,7 +237,7 @@ public function register(Container $container): void
                 //     tags:
                 //       - { name: foo_bar }
                 // @endcode
-                if (!isset($serviceDefinition['class']) && class_exists($serviceId)) {
+                if (!isset($serviceDefinition['class']) && $this->classExists($serviceId)) {
                     $serviceDefinition['class'] = $serviceId;
                 }
                 // @todo sanitize "calls" and "configurator" and "factory"
@@ -419,4 +419,16 @@ protected function camelize(string $id): string
     {
         return strtr(ucwords(strtr($id, ['_' => ' ', '.' => '_ ', '\\' => '_ '])), [' ' => '']);
     }
+
+    private function classExists(string $className): bool
+    {
+        try {
+            return class_exists($className);
+        } catch (Throwable) {
+            // Loading the class can fail when it depends on a class from an
+            // extension that is not available, such as a decorator for an
+            // optional module registered with decoration_on_invalid: ignore.
+            return false;
+        }
+    }
 }

src/Drupal/ServiceMap.php

@@ -2,6 +2,7 @@
 
 namespace mglaman\PHPStanDrupal\Drupal;
 
+use Throwable;
 use function class_exists;
 
 class ServiceMap
@@ -36,12 +37,12 @@ public function setDrupalServices(array $drupalServices): void
             }
 
             if (isset($serviceDefinition['decorates'])) {
-                $decorators[$serviceDefinition['decorates']][] = $serviceId;
+                $decorators[$serviceDefinition['decorates']][$serviceId] = $serviceDefinition['decoration_on_invalid'] ?? 'exception';
             }
 
             // @todo support factories
             if (!isset($serviceDefinition['class'])) {
-                if (class_exists($serviceId)) {
+                if (self::classExists($serviceId)) {
                     $serviceDefinition['class'] = $serviceId;
                 } else {
                     continue;
@@ -67,15 +68,36 @@ public function setDrupalServices(array $drupalServices): void
         }
 
         foreach ($decorators as $decorated_service_id => $services) {
-            foreach ($services as $dcorating_service_id) {
+            foreach ($services as $decorating_service_id => $decoration_on_invalid) {
                 if (!isset(self::$services[$decorated_service_id])) {
+                    // Symfony removes the decorating service from the
+                    // container when the decorated service does not exist and
+                    // decoration_on_invalid is set to ignore.
+                    if ($decoration_on_invalid === 'ignore') {
+                        unset(self::$services[$decorating_service_id]);
+                    }
                     continue;
                 }
-                self::$services[$decorated_service_id]->addDecorator(self::$services[$dcorating_service_id]);
+                if (!isset(self::$services[$decorating_service_id])) {
+                    continue;
+                }
+                self::$services[$decorated_service_id]->addDecorator(self::$services[$decorating_service_id]);
             }
         }
     }
 
+    private static function classExists(string $className): bool
+    {
+        try {
+            return class_exists($className);
+        } catch (Throwable) {
+            // Loading the class can fail when it depends on a class from an
+            // extension that is not available, such as a decorator for an
+            // optional module registered with decoration_on_invalid: ignore.
+            return false;
+        }
+    }
+
     private function resolveParentDefinition(string $parentId, array $serviceDefinition, array $drupalServices): array
     {
         $parentDefinition = $drupalServices[$parentId] ?? [];

tests/fixtures/drupal/modules/service_map_broken/service_map_broken.info.yml

@@ -0,0 +1,3 @@
+name: service_map_broken
+type: module
+core_version_requirement: ^8 || ^9

tests/fixtures/drupal/modules/service_map_broken/service_map_broken.services.yml

@@ -0,0 +1,4 @@
+services:
+  Drupal\service_map_broken\ExtendsMissingClass:
+    decorates: missing_module.render_converter
+    decoration_on_invalid: ignore

tests/fixtures/drupal/modules/service_map_broken/src/ExtendsMissingClass.php

@@ -0,0 +1,14 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\service_map_broken;
+
+use Drupal\missing_module\RenderConverter;
+
+// The parent class belongs to a module that is not available. Declaring this
+// class throws an Error, which the service map must survive.
+final class ExtendsMissingClass extends RenderConverter
+{
+
+}

tests/src/ServiceMapFactoryTest.php

@@ -112,10 +112,48 @@ public function testFactory(string $id, callable $validator): void
                 'decorates' => 'unknown',
                 'class' => 'Drupal\service_map\Override',
             ],
+            'ignored_decorator_of_unknown_service' => [
+                'decorates' => 'unknown',
+                'decoration_on_invalid' => 'ignore',
+                'class' => 'Drupal\service_map\Override',
+            ],
         ]);
         $validator($service->getService($id));
     }
 
+    /**
+     * @covers \mglaman\PHPStanDrupal\Drupal\ServiceMap::setDrupalServices
+     * @covers \mglaman\PHPStanDrupal\Drupal\ServiceMap::getService
+     * @covers \mglaman\PHPStanDrupal\Drupal\ServiceMap::classExists
+     */
+    public function testShorthandServiceWithUnloadableClass(): void
+    {
+        $autoloader = static function (string $class): void {
+            if ($class === 'Drupal\service_map_broken\ExtendsMissingClass') {
+                require __DIR__ . '/../fixtures/drupal/modules/service_map_broken/src/ExtendsMissingClass.php';
+            }
+        };
+        spl_autoload_register($autoloader);
+        try {
+            $service = new ServiceMap();
+            $service->setDrupalServices([
+                'decorated_service' => [
+                    'class' => 'Drupal\service_map\Base',
+                ],
+                'Drupal\service_map_broken\ExtendsMissingClass' => [
+                    'decorates' => 'decorated_service',
+                    'decoration_on_invalid' => 'ignore',
+                ],
+            ]);
+        } finally {
+            spl_autoload_unregister($autoloader);
+        }
+        self::assertNull($service->getService('Drupal\service_map_broken\ExtendsMissingClass'));
+        $decorated = $service->getService('decorated_service');
+        self::assertNotNull($decorated);
+        self::assertCount(0, $decorated->getDecorators());
+    }
+
     public static function getServiceProvider(): \Iterator
     {
         yield [
@@ -230,6 +268,18 @@ function (DrupalServiceDefinition $service): void {
                 self::assertEquals(LoggerChannel::class, $service->getClass());
             }
         ];
+        yield [
+            'decorating_an_unknown_service',
+            function (?DrupalServiceDefinition $service): void {
+                self::assertNotNull($service, 'decorating_an_unknown_service');
+            },
+        ];
+        yield [
+            'ignored_decorator_of_unknown_service',
+            function (?DrupalServiceDefinition $service): void {
+                self::assertNull($service, 'ignored_decorator_of_unknown_service');
+            },
+        ];
         yield [
             'service_map.base_to_be_decorated',
             function (DrupalServiceDefinition $service): void {