Fixed #36407 -- Ensured default value is cast in Case expressions used in ORDER BY clause.

Thanks to deceze for the report. Thanks to Sarah Boyce for the test.
Thanks to Simon Charette for the investigation and review.

django/db/models/expressions.py

@@ -1711,14 +1711,24 @@ class Case(SQLiteNumericMixin, Expression):
             except EmptyResultSet:
                 continue
             except FullResultSet:
-                default_sql, default_params = compiler.compile(case.result)
+                default = case.result
                 break
             case_parts.append(case_sql)
             sql_params.extend(case_params)
         else:
-            default_sql, default_params = compiler.compile(self.default)
-        if not case_parts:
-            return default_sql, default_params
+            default = self.default
+        if case_parts:
+            default_sql, default_params = compiler.compile(default)
+        else:
+            if (
+                isinstance(default, Value)
+                and (output_field := default._output_field_or_none) is not None
+            ):
+                from django.db.models.functions import Cast
+
+                default = Cast(default, output_field)
+            return compiler.compile(default)
+
         case_joiner = case_joiner or self.case_joiner
         template_params["cases"] = case_joiner.join(case_parts)
         template_params["default"] = default_sql

tests/ordering/tests.py

@@ -3,15 +3,18 @@ from operator import attrgetter
 
 from django.core.exceptions import FieldError
 from django.db.models import (
+    Case,
     CharField,
     Count,
     DateTimeField,
     F,
+    IntegerField,
     Max,
     OrderBy,
     OuterRef,
     Subquery,
     Value,
+    When,
 )
 from django.db.models.functions import Length, Upper
 from django.test import TestCase
@@ -526,6 +529,17 @@ class OrderingTests(TestCase):
         qs = Article.objects.order_by(Value("1", output_field=CharField()), "-headline")
         self.assertSequenceEqual(qs, [self.a4, self.a3, self.a2, self.a1])
 
+    def test_order_by_case_when_constant_value(self):
+        qs = Article.objects.order_by(
+            Case(
+                When(pk__in=[], then=Value(1)),
+                default=Value(0),
+                output_field=IntegerField(),
+            ).desc(),
+            "pk",
+        )
+        self.assertSequenceEqual(qs, [self.a1, self.a2, self.a3, self.a4])
+
     def test_related_ordering_duplicate_table_reference(self):
         """
         An ordering referencing a model with an ordering referencing a model