Fixed #30446 -- Resolved Value.output_field for stdlib types.

This required implementing a limited form of dynamic dispatch to combine
expressions with numerical output. Refs #26355 should eventually provide
a better interface for that.

tests/aggregation/tests.py

@@ -848,10 +848,6 @@ class AggregateTestCase(TestCase):
         book = Book.objects.annotate(val=Max(2, output_field=IntegerField())).first()
         self.assertEqual(book.val, 2)
 
-    def test_missing_output_field_raises_error(self):
-        with self.assertRaisesMessage(FieldError, 'Cannot resolve expression type, unknown output_field'):
-            Book.objects.annotate(val=Max(2)).first()
-
     def test_annotation_expressions(self):
         authors = Author.objects.annotate(combined_ages=Sum(F('age') + F('friends__age'))).order_by('name')
         authors2 = Author.objects.annotate(combined_ages=Sum('age') + Sum('friends__age')).order_by('name')
@@ -893,7 +889,7 @@ class AggregateTestCase(TestCase):
 
     def test_combine_different_types(self):
         msg = (
-            'Expression contains mixed types: FloatField, IntegerField. '
+            'Expression contains mixed types: FloatField, DecimalField. '
             'You must set output_field.'
         )
         qs = Book.objects.annotate(sums=Sum('rating') + Sum('pages') + Sum('price'))

tests/aggregation_regress/tests.py

@@ -388,7 +388,7 @@ class AggregationTests(TestCase):
         )
 
     def test_annotated_conditional_aggregate(self):
-        annotated_qs = Book.objects.annotate(discount_price=F('price') * 0.75)
+        annotated_qs = Book.objects.annotate(discount_price=F('price') * Decimal('0.75'))
         self.assertAlmostEqual(
             annotated_qs.aggregate(test=Avg(Case(
                 When(pages__lt=400, then='discount_price'),

tests/expressions/tests.py

@@ -3,15 +3,17 @@ import pickle
 import unittest
 import uuid
 from copy import deepcopy
+from decimal import Decimal
 from unittest import mock
 
 from django.core.exceptions import FieldError
 from django.db import DatabaseError, NotSupportedError, connection
 from django.db.models import (
-    Avg, BooleanField, Case, CharField, Count, DateField, DateTimeField,
-    DurationField, Exists, Expression, ExpressionList, ExpressionWrapper, F,
-    Func, IntegerField, Max, Min, Model, OrderBy, OuterRef, Q, StdDev,
-    Subquery, Sum, TimeField, UUIDField, Value, Variance, When,
+    Avg, BinaryField, BooleanField, Case, CharField, Count, DateField,
+    DateTimeField, DecimalField, DurationField, Exists, Expression,
+    ExpressionList, ExpressionWrapper, F, FloatField, Func, IntegerField, Max,
+    Min, Model, OrderBy, OuterRef, Q, StdDev, Subquery, Sum, TimeField,
+    UUIDField, Value, Variance, When,
 )
 from django.db.models.expressions import Col, Combinable, Random, RawSQL, Ref
 from django.db.models.functions import (
@@ -1711,6 +1713,30 @@ class ValueTests(TestCase):
         value = Value('foo', output_field=CharField())
         self.assertEqual(value.as_sql(compiler, connection), ('%s', ['foo']))
 
+    def test_resolve_output_field(self):
+        value_types = [
+            ('str', CharField),
+            (True, BooleanField),
+            (42, IntegerField),
+            (3.14, FloatField),
+            (datetime.date(2019, 5, 15), DateField),
+            (datetime.datetime(2019, 5, 15), DateTimeField),
+            (datetime.time(3, 16), TimeField),
+            (datetime.timedelta(1), DurationField),
+            (Decimal('3.14'), DecimalField),
+            (b'', BinaryField),
+            (uuid.uuid4(), UUIDField),
+        ]
+        for value, ouput_field_type in value_types:
+            with self.subTest(type=type(value)):
+                expr = Value(value)
+                self.assertIsInstance(expr.output_field, ouput_field_type)
+
+    def test_resolve_output_field_failure(self):
+        msg = 'Cannot resolve expression type, unknown output_field'
+        with self.assertRaisesMessage(FieldError, msg):
+            Value(object()).output_field
+
 
 class FieldTransformTests(TestCase):
 
@@ -1848,7 +1874,9 @@ class ExpressionWrapperTests(SimpleTestCase):
         self.assertEqual(expr.get_group_by_cols(alias=None), [])
 
     def test_non_empty_group_by(self):
-        expr = ExpressionWrapper(Lower(Value('f')), output_field=IntegerField())
+        value = Value('f')
+        value.output_field = None
+        expr = ExpressionWrapper(Lower(value), output_field=IntegerField())
         group_by_cols = expr.get_group_by_cols(alias=None)
         self.assertEqual(group_by_cols, [expr.expression])
         self.assertEqual(group_by_cols[0].output_field, expr.output_field)

tests/ordering/tests.py

@@ -1,7 +1,6 @@
 from datetime import datetime
 from operator import attrgetter
 
-from django.core.exceptions import FieldError
 from django.db.models import (
     CharField, DateTimeField, F, Max, OuterRef, Subquery, Value,
 )
@@ -439,17 +438,6 @@ 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_constant_value_without_output_field(self):
-        msg = 'Cannot resolve expression type, unknown output_field'
-        qs = Article.objects.annotate(constant=Value('1')).order_by('constant')
-        for ordered_qs in (
-            qs,
-            qs.values('headline'),
-            Article.objects.order_by(Value('1')),
-        ):
-            with self.subTest(ordered_qs=ordered_qs), self.assertRaisesMessage(FieldError, msg):
-                ordered_qs.first()
-
     def test_related_ordering_duplicate_table_reference(self):
         """
         An ordering referencing a model with an ordering referencing a model