IN queries inside conditional case expressions crashes on django 1.8

[Alberto Avila]

When making a IN filter expression inside a conditional case expression django crashes with the following exception:

AttributeError: 'SubqueryConstraint' object has no attribute 'lhs'

The stacktrace looks like:

    lib/python2.7/site-packages/django/db/models/query.py line 794 in annotate
      obj.query.add_annotation(annotation, alias, is_summary=False)
    lib/python2.7/site-packages/django/db/models/sql/query.py line 983 in add_annotation
      summarize=is_summary)
    lib/python2.7/site-packages/django/db/models/aggregates.py line 21 in resolve_expression
      if c.source_expressions[0].contains_aggregate and not summarize:
    lib/python2.7/site-packages/django/utils/functional.py line 59 in __get__
      res = instance.__dict__[self.name] = self.func(instance)
    lib/python2.7/site-packages/django/db/models/expressions.py line 179 in contains_aggregate
      if expr and expr.contains_aggregate:
    lib/python2.7/site-packages/django/utils/functional.py line 59 in __get__
      res = instance.__dict__[self.name] = self.func(instance)
    lib/python2.7/site-packages/django/db/models/expressions.py line 179 in contains_aggregate
      if expr and expr.contains_aggregate:
    lib/python2.7/site-packages/django/utils/functional.py line 59 in __get__
      res = instance.__dict__[self.name] = self.func(instance)
    lib/python2.7/site-packages/django/db/models/sql/where.py line 324 in contains_aggregate
      return self._contains_aggregate(self)
    lib/python2.7/site-packages/django/db/models/sql/where.py line 320 in _contains_aggregate
      return any(cls._contains_aggregate(c) for c in obj.children)
    lib/python2.7/site-packages/django/db/models/sql/where.py line 320 in <genexpr>
      return any(cls._contains_aggregate(c) for c in obj.children)
    lib/python2.7/site-packages/django/db/models/sql/where.py line 319 in _contains_aggregate
      return getattr(obj.lhs, 'contains_aggregate', False) or getattr(obj.rhs, 'contains_aggregate', False)
   AttributeError: 'SubqueryConstraint' object has no attribute 'lhs'

This error is already fixed on django 1.9 branch. The problem was fixed with this commit:

​https://github.com/django/django/commit/afe0bb7b13bb8dc4370f32225238012c873b0ee3#diff-73bc0dcb1a1c931adcf1d344abca22ffR180

I'll create a PR with the changes from that commit that fixes the issue, not sure if the entire commit can be (or should be) backported to 1.8

UPDATE: PR with only the changes needed and its test: ​https://github.com/django/django/pull/5967