[3.0.x] Fixed #31607 -- Fixed evaluated Subquery equality.

Regression in 691def10a0.

Backport of a125da6a7c from master

django/db/models/expressions.py

@@ -1040,11 +1040,18 @@ class Subquery(Expression):
     def __init__(self, queryset, output_field=None, **extra):
         self.query = queryset.query
         self.extra = extra
+        # Prevent the QuerySet from being evaluated.
+        self.queryset = queryset._chain(_result_cache=[], prefetch_done=True)
         super().__init__(output_field)
 
     def __getstate__(self):
         state = super().__getstate__()
-        state.pop('_constructor_args', None)
+        args, kwargs = state['_constructor_args']
+        if args:
+            args = (self.queryset, *args[1:])
+        else:
+            kwargs['queryset'] = self.queryset
+        state['_constructor_args'] = args, kwargs
         return state
 
     def get_source_expressions(self):

tests/expressions/tests.py

@@ -512,6 +512,25 @@ class BasicExpressionsTests(TestCase):
             Employee.objects.exclude(company_point_of_contact_set=None).values('pk')
         )
 
+    def test_subquery_eq(self):
+        qs = Employee.objects.annotate(
+            is_ceo=Exists(Company.objects.filter(ceo=OuterRef('pk'))),
+            is_point_of_contact=Exists(
+                Company.objects.filter(point_of_contact=OuterRef('pk')),
+            ),
+            small_company=Exists(
+                queryset=Company.objects.filter(num_employees__lt=200),
+            ),
+        ).filter(is_ceo=True, is_point_of_contact=False, small_company=True)
+        self.assertNotEqual(
+            qs.query.annotations['is_ceo'],
+            qs.query.annotations['is_point_of_contact'],
+        )
+        self.assertNotEqual(
+            qs.query.annotations['is_ceo'],
+            qs.query.annotations['small_company'],
+        )
+
     def test_in_subquery(self):
         # This is a contrived test (and you really wouldn't write this query),
         # but it is a succinct way to test the __in=Subquery() construct.