[5.0.x] Refs #30446, Refs #34944 -- Fixed crash when adding GeneratedField with string Value().

This should allow smarter output_field inferring in functions dealing
with text expressions.

Regression in f333e3513e8bdf5ffeb6eeb63021c230082e6f95.

Backport of 8b1acc0440418ac8f45ba48e2dfcf5126c83341b from main

3 files changed, 19 insertions, 1 deletions

diff --git a/django/db/models/expressions.py b/django/db/models/expressions.py
index b38f8f64ef..d38093bfed 100644
--- a/django/db/models/expressions.py
+++ b/django/db/models/expressions.py
@@ -1081,7 +1081,7 @@ class Value(SQLiteNumericMixin, Expression):
 
     def _resolve_output_field(self):
         if isinstance(self.value, str):
-            return fields.CharField()
+            return fields.CharField(max_length=len(self.value))
         if isinstance(self.value, bool):
             return fields.BooleanField()
         if isinstance(self.value, int):
diff --git a/tests/expressions/tests.py b/tests/expressions/tests.py
index 073e2e9258..9743d024b1 100644
--- a/tests/expressions/tests.py
+++ b/tests/expressions/tests.py
@@ -2227,6 +2227,7 @@ class ValueTests(TestCase):
             with self.subTest(type=type(value)):
                 expr = Value(value)
                 self.assertIsInstance(expr.output_field, output_field_type)
+        self.assertEqual(Value("foo").output_field.max_length, 3)
 
     def test_resolve_output_field_failure(self):
         msg = "Cannot resolve expression type, unknown output_field"
diff --git a/tests/schema/tests.py b/tests/schema/tests.py
index e1e3c7d3c6..5488013bfc 100644
--- a/tests/schema/tests.py
+++ b/tests/schema/tests.py
@@ -850,6 +850,23 @@ class SchemaTests(TransactionTestCase):
 
     @isolate_apps("schema")
     @skipUnlessDBFeature("supports_stored_generated_columns")
+    def test_add_generated_field_with_string_value(self):
+        class GeneratedFieldStringValueModel(Model):
+            value = GeneratedField(expression=Value("static string"), db_persist=True)
+
+            class Meta:
+                app_label = "schema"
+
+        with CaptureQueriesContext(connection) as ctx:
+            with connection.schema_editor() as editor:
+                editor.create_model(GeneratedFieldStringValueModel)
+        self.assertIs(
+            any("None" in query["sql"] for query in ctx.captured_queries),
+            False,
+        )
+
+    @isolate_apps("schema")
+    @skipUnlessDBFeature("supports_stored_generated_columns")
     def test_add_generated_field_with_output_field(self):
         class GeneratedFieldOutputFieldModel(Model):
             price = DecimalField(max_digits=7, decimal_places=2)