Infer return type from class-string<T> argument in ClassResolver extensions

When a variable typed as class-string<T> is passed to
ClassResolverInterface::getInstanceFromDefinition() or Drupal::classResolver(),
PHPStan now infers the return type as T instead of falling back to object.

The existing service-ID and constant-string class-name resolution via ServiceMap
is unchanged. This only fills the gap for non-constant class-string<T> variables.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: mglaman

src/Type/DrupalClassResolverReturnType.php

@@ -11,6 +11,7 @@
 use PHPStan\Reflection\MethodReflection;
 use PHPStan\Reflection\ParametersAcceptorSelector;
 use PHPStan\Type\ObjectType;
+use PHPStan\Type\ObjectWithoutClassType;
 use PHPStan\Type\Type;
 use function count;
 
@@ -25,6 +26,12 @@ public static function getType(
     ): Type {
         $arg1 = $scope->getType($methodCall->getArgs()[0]->value);
         if (count($arg1->getConstantStrings()) === 0) {
+            // Handle class-string<T> typed variables
+            $classStringObjectType = $arg1->getClassStringObjectType();
+            if (!$classStringObjectType instanceof ObjectWithoutClassType) {
+                return $classStringObjectType;
+            }
+
             return ParametersAcceptorSelector::selectFromArgs(
                 $scope,
                 $methodCall->getArgs(),

tests/src/Type/data/drupal-class-resolver.php

@@ -8,6 +8,22 @@
 
 class Foo {}
 
+function test_class_string_vars(): void {
+    /** @var class-string<Foo> $fooClass */
+    $fooClass = Foo::class;
+    assertType(Foo::class, (new ClassResolver())->getInstanceFromDefinition($fooClass));
+    assertType(Foo::class, \Drupal::service('class_resolver')->getInstanceFromDefinition($fooClass));
+    assertType(Foo::class, \Drupal::classResolver()->getInstanceFromDefinition($fooClass));
+    assertType(Foo::class, \Drupal::classResolver($fooClass));
+
+    /** @var class-string<MyService> $serviceClass */
+    $serviceClass = MyService::class;
+    assertType(MyService::class, (new ClassResolver())->getInstanceFromDefinition($serviceClass));
+    assertType(MyService::class, \Drupal::service('class_resolver')->getInstanceFromDefinition($serviceClass));
+    assertType(MyService::class, \Drupal::classResolver()->getInstanceFromDefinition($serviceClass));
+    assertType(MyService::class, \Drupal::classResolver($serviceClass));
+}
+
 function test(): void {
     assertType(Foo::class, (new ClassResolver())->getInstanceFromDefinition(Foo::class));
     assertType(Foo::class, \Drupal::service('class_resolver')->getInstanceFromDefinition(Foo::class));