Refs #25425 -- Allowed unresolved Value() instances to be compiled.

Previously unresolved Value() instances were only allowed to be
compiled if they weren't initialized with an output_field.

Given the usage of unresolved Value() instances is relatively common in
as_sql() overrides it's less controversial to add explicit support for
this previously undefined behavior now and revisit whether or not it
should be deprecated in the future.

django/db/models/expressions.py

@@ -676,6 +676,10 @@ class Func(SQLiteNumericMixin, Expression):
 
 class Value(Expression):
     """Represent a wrapped value as a node within an expression."""
+    # Provide a default value for `for_save` in order to allow unresolved
+    # instances to be compiled until a decision is taken in #25425.
+    for_save = False
+
     def __init__(self, value, output_field=None):
         """
         Arguments:

tests/expressions/tests.py

@@ -1703,6 +1703,14 @@ class ValueTests(TestCase):
         with self.assertRaisesMessage(ValueError, msg):
             ExpressionList()
 
+    def test_compile_unresolved(self):
+        # This test might need to be revisited later on if #25425 is enforced.
+        compiler = Time.objects.all().query.get_compiler(connection=connection)
+        value = Value('foo')
+        self.assertEqual(value.as_sql(compiler, connection), ('%s', ['foo']))
+        value = Value('foo', output_field=CharField())
+        self.assertEqual(value.as_sql(compiler, connection), ('%s', ['foo']))
+
 
 class FieldTransformTests(TestCase):