Cannot get auto-completion from type hinting only

This is a trivial example, but I cannot get it to work properly:

```php
<?php

class Argument
{
  public $property1;
  public $property2;
}

class TryMe
{
  public function method(Argument $argument)
  {
    $argument-><|>
  }
}
```

When triggering omni-completion `<C-X><C-O>` at the cursor position `<|>`, I get all sorts of unrelated entries. If the line above the cursor is annotated with a `@var` comment block, the auto-completion correctly lists only members of the `Argument` class:

```php
<?php

class Argument
{
  public $property1;
  public $property2;
}

class TryMe
{
  public function method(Argument $argument)
  {
    /* @var $argument Argument */
    $argument-><|>
  }
}
```

Is this a bug or a limitation? Is there a workaround?