Generic object factory should be constrained to return an object

[bdsl]

Psalm currently reports no issues with the following code:

class GenericObjectFactory 
{
   /**
    * @psalm-template T
    * @psalm-param class-string<T> $type
    * @psalm-return T
    */
    public function getObject(string $type)
    {
        return 3;
    }
}

But since we have a class-string<T>, I think psalm could know that T must be an object type, and so interpret the code as equivalent to the following, for which it does report InvalidReturnStatement and InvalidReturnType:

<?php

class GenericObjectFactory 
{
   /**
    * @psalm-template T of object
    * @psalm-param class-string<T> $type
    * @psalm-return T
    */
    public function getObject(string $type)
    {
        return 3;
    }
}

This is motivated by a docblock I added to jms/serializer, and the hope that implementing this feature would catch possible errors in implementations of that interface.

[bdsl]

Thanks!