add test for build error special values

tests/test_basic.py

@@ -1131,6 +1131,23 @@ def test_build_error_handler_reraise():
         pytest.raises(BuildError, flask.url_for, 'not.existing')
 
 
+def test_url_for_passes_special_values_to_build_error_handler():
+    app = flask.Flask(__name__)
+
+    @app.url_build_error_handlers.append
+    def handler(error, endpoint, values):
+        assert values == {
+            '_external': False,
+            '_anchor': None,
+            '_method': None,
+            '_scheme': None,
+        }
+        return 'handled'
+
+    with app.test_request_context():
+        flask.url_for('/')
+
+
 def test_custom_converters():
     from werkzeug.routing import BaseConverter