Fixed #36198 -- Implemented unresolved transform expression replacement.

This allows the proper resolving of F("field__transform") when
performing constraint validation.

Thanks Tom Hall for the report and Sarah for the test.

tests/expressions/tests.py

@@ -58,10 +58,12 @@ from django.db.models.expressions import (
 from django.db.models.functions import (
     Coalesce,
     Concat,
+    ExtractDay,
     Left,
     Length,
     Lower,
     Substr,
+    TruncDate,
     Upper,
 )
 from django.db.models.sql import constants
@@ -1330,6 +1332,38 @@ class FTests(SimpleTestCase):
         with self.assertRaisesMessage(TypeError, msg):
             "" in F("name")
 
+    def test_replace_expressions_transform(self):
+        replacements = {F("timestamp"): Value(None)}
+        transform_ref = F("timestamp__date")
+        self.assertIs(transform_ref.replace_expressions(replacements), transform_ref)
+        invalid_transform_ref = F("timestamp__invalid")
+        self.assertIs(
+            invalid_transform_ref.replace_expressions(replacements),
+            invalid_transform_ref,
+        )
+        replacements = {F("timestamp"): Value(datetime.datetime(2025, 3, 1, 14, 10))}
+        self.assertEqual(
+            F("timestamp__date").replace_expressions(replacements),
+            TruncDate(Value(datetime.datetime(2025, 3, 1, 14, 10))),
+        )
+        self.assertEqual(
+            F("timestamp__date__day").replace_expressions(replacements),
+            ExtractDay(TruncDate(Value(datetime.datetime(2025, 3, 1, 14, 10)))),
+        )
+        invalid_nested_transform_ref = F("timestamp__date__invalid")
+        self.assertIs(
+            invalid_nested_transform_ref.replace_expressions(replacements),
+            invalid_nested_transform_ref,
+        )
+        # `replacements` is not unnecessarily looked up a second time for
+        # transform-less field references as it's the case the vast majority of
+        # the time.
+        mock_replacements = mock.Mock()
+        mock_replacements.get.return_value = None
+        field_ref = F("name")
+        self.assertIs(field_ref.replace_expressions(mock_replacements), field_ref)
+        mock_replacements.get.assert_called_once_with(field_ref)
+
 
 class ExpressionsTests(TestCase):
     def test_F_reuse(self):