Fixed #29330 -- Fixed crash when pickling BaseExpression.

Regression in cfff2af02b.

django/db/models/expressions.py

@@ -153,6 +153,11 @@ class BaseExpression:
         if output_field is not None:
             self.output_field = output_field
 
+    def __getstate__(self):
+        state = self.__dict__.copy()
+        state.pop('convert_value', None)
+        return state
+
     def get_db_converters(self, connection):
         return (
             []

tests/expressions/tests.py

@@ -1,4 +1,5 @@
 import datetime
+import pickle
 import unittest
 import uuid
 from copy import deepcopy
@@ -585,6 +586,11 @@ class BasicExpressionsTests(TestCase):
         outer = Company.objects.filter(pk__in=Subquery(inner.values('pk')))
         self.assertEqual(outer.get().name, 'Test GmbH')
 
+    def test_pickle_expression(self):
+        expr = Value(1, output_field=models.IntegerField())
+        expr.convert_value  # populate cached property
+        self.assertEqual(pickle.loads(pickle.dumps(expr)), expr)
+
 
 class IterableLookupInnerExpressionsTests(TestCase):
     @classmethod