Hi Steve,

I just uploaded HHH_1830.zip containing a TestCase.
It contains a simple parent-child relationship.
(I've made one little mistake with the sample data: child.setParent(parent) is missing, but that's not important for reproducing the bug)

The query string is:
SELECT p.id, size(descendants) from Parent p left outer join p.children descendants group by p.id.

With 4.3.11, the following SQL is being generated and everything is well:

   select
        parent0_.id as col_0_0_,
        count(children1_.parent_id) as col_1_0_ 
    from
        Parent parent0_ 
    left outer join
        Child children1_ 
            on parent0_.id=children1_.parent_id 
    group by
        parent0_.id

But with 5.2.2, the aforementioned Exception occurs.

Please also note that the workaround provided by Marius Capris is working, but produces the following ugly sql:
Query: SELECT p.id, size( p.children) from Parent p left outer join p.children descendants group by p.id

    select
        parent0_.id as col_0_0_,
        count(children2_.parent_id) as col_1_0_ 
    from
        Parent parent0_ 
    left outer join
        Child children1_ 
            on parent0_.id=children1_.parent_id,
        Child children2_ 
    where
        parent0_.id=children2_.parent_id 
    group by
        parent0_.id

I hope this helps hunting that bug.

Regards
Matthias