Fix __typename field handling to use GraphQLString

ariadne_codegen/client_generators/result_types.py

@@ -13,8 +13,8 @@
     GraphQLNamedType,
     GraphQLNonNull,
     GraphQLObjectType,
-    GraphQLScalarType,
     GraphQLSchema,
+    GraphQLString,
     GraphQLUnionType,
     InlineFragmentNode,
     NameNode,
@@ -423,7 +423,7 @@ def _get_field_from_schema(self, type_name: str, field_name: str) -> GraphQLFiel
         except KeyError as exc:
             if field_name == TYPENAME_FIELD_NAME:
                 return GraphQLField(
-                    type_=GraphQLNonNull(type_=GraphQLScalarType(name="String"))
+                    type_=GraphQLNonNull(type_=GraphQLString)
                 )
             raise ParsingError(
                 f"Field {field_name} not found in type {type_name}."

tests/client_generators/result_types_generator/test_include_typename.py

@@ -282,3 +282,50 @@ def test_union_discriminator_respects_include_typename():
     assert (
         not has_discriminator_without_typename
     ), "Should not have discriminator when include_typename=False"
+
+
+def test_get_field_from_schema_handles_typename_correctly():
+    """Test that _get_field_from_schema correctly handles __typename field."""
+    from graphql import GraphQLNonNull, GraphQLString
+
+    from ariadne_codegen.exceptions import ParsingError
+
+    schema = build_schema(SIMPLE_SCHEMA)
+
+    query_str = """
+        query GetUser {
+            user {
+                id
+            }
+        }
+    """
+
+    document = parse(query_str)
+    operation = document.definitions[0]
+    assert isinstance(operation, OperationDefinitionNode)
+
+    generator = ResultTypesGenerator(
+        schema=schema,
+        operation_definition=operation,
+        enums_module_name="enums",
+        include_typename=True,
+    )
+
+    # __typename is not in the schema's type_map fields, so it should be handled
+    # as a special case. This validates the fix where using GraphQLString instead
+    # of GraphQLScalarType(name="String") prevents "Redefinition of reserved type" error.
+    typename_field = generator._get_field_from_schema("User", TYPENAME_FIELD_NAME)
+
+    # Verify the field was created correctly using the built-in GraphQLString type
+    assert typename_field is not None
+    assert isinstance(typename_field.type, GraphQLNonNull)
+    assert typename_field.type.of_type is GraphQLString
+    assert typename_field.type.of_type.name == "String"
+
+    # Test that requesting a non-existent field (other than __typename) raises an error
+    try:
+        generator._get_field_from_schema("User", "nonExistentField")
+        assert False, "Expected ParsingError for non-existent field"
+    except ParsingError as e:
+        assert "nonExistentField" in str(e)
+        assert "User" in str(e)