title: Receiver type is forward declaration
body: <p>I have this code (along with other stuff):</p>
  
  <pre><code>- (NSManagedObjectContext *) managedObjectContext
  {
      assert([NSThread isMainThread]);
      if (_managedObjectContext != nil) {
          return _managedObjectContext;
      }
      NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
      if (coordinator != nil) {
          _managedObjectContext = [[NSManagedObjectContext alloc] init];
          [_managedObjectContext setPersistentStoreCoordinator: coordinator];
      }
  
      return _managedObjectContext;
  }
  </code></pre>
  
  <p>These lines are giving me errors that class message is a forward declaration:</p>
  
  <pre><code>_managedObjectContext = [[NSManagedObjectContext alloc] init];
  [_managedObjectContext setPersistentStoreCoordinator: coordinator];
  </code></pre>
  
  <p>What is this and how do I fix it?</p>