Fixed #36051 -- Declared arity on aggregate functions.

Follow-up to 4a66a69239.

tests/aggregation/tests.py

@@ -1276,6 +1276,8 @@ class AggregateTestCase(TestCase):
             Book.objects.annotate(Max("id")).annotate(Sum("id__max"))
 
         class MyMax(Max):
+            arity = None
+
             def as_sql(self, compiler, connection):
                 self.set_source_expressions(self.get_source_expressions()[0:1])
                 return super().as_sql(compiler, connection)
@@ -1288,6 +1290,7 @@ class AggregateTestCase(TestCase):
     def test_multi_arg_aggregate(self):
         class MyMax(Max):
             output_field = DecimalField()
+            arity = None
 
             def as_sql(self, compiler, connection):
                 copy = self.copy()
@@ -2178,6 +2181,27 @@ class AggregateTestCase(TestCase):
         )
         self.assertEqual(list(author_qs), [337])
 
+    def test_aggregate_arity(self):
+        funcs_with_inherited_constructors = [Avg, Max, Min, Sum]
+        msg = "takes exactly 1 argument (2 given)"
+        for function in funcs_with_inherited_constructors:
+            with (
+                self.subTest(function=function),
+                self.assertRaisesMessage(TypeError, msg),
+            ):
+                function(Value(1), Value(2))
+
+        funcs_with_custom_constructors = [Count, StdDev, Variance]
+        for function in funcs_with_custom_constructors:
+            with self.subTest(function=function):
+                # Extra arguments are rejected via the constructor.
+                with self.assertRaises(TypeError):
+                    function(Value(1), True, Value(2))
+                # If the constructor is skipped, the arity check runs.
+                func_instance = function(Value(1), True)
+                with self.assertRaisesMessage(TypeError, msg):
+                    super(function, func_instance).__init__(Value(1), Value(2))
+
 
 class AggregateAnnotationPruningTests(TestCase):
     @classmethod