fix typing that wasn't available in Python 3.6.0

CHANGES.rst

@@ -12,6 +12,7 @@ Unreleased
     imports in user projects. :issue:`4024`
 -   Fix type annotation for ``g`` and inform mypy that it is a namespace
     object that has arbitrary attributes. :issue:`4020`
+-   Fix some types that weren't available in Python 3.6.0. :issue:`4040`
 
 
 Version 2.0.0

src/flask/app.py

@@ -72,6 +72,7 @@ from .wrappers import Request
 from .wrappers import Response
 
 if t.TYPE_CHECKING:
+    import typing_extensions as te
     from .blueprints import Blueprint
     from .testing import FlaskClient
     from .testing import FlaskCliRunner
@@ -1441,7 +1442,7 @@ class Flask(Scaffold):
             f"Exception on {request.path} [{request.method}]", exc_info=exc_info
         )
 
-    def raise_routing_exception(self, request: Request) -> t.NoReturn:
+    def raise_routing_exception(self, request: Request) -> "te.NoReturn":
         """Exceptions that are recording during routing are reraised with
         this method.  During debug we are not reraising redirect requests
         for non ``GET``, ``HEAD``, or ``OPTIONS`` requests and we're raising

src/flask/sessions.py

@@ -12,6 +12,7 @@ from .helpers import is_ip
 from .json.tag import TaggedJSONSerializer
 
 if t.TYPE_CHECKING:
+    import typing_extensions as te
     from .app import Flask
     from .wrappers import Request, Response
 
@@ -92,7 +93,7 @@ class NullSession(SecureCookieSession):
     but fail on setting.
     """
 
-    def _fail(self, *args: t.Any, **kwargs: t.Any) -> t.NoReturn:
+    def _fail(self, *args: t.Any, **kwargs: t.Any) -> "te.NoReturn":
         raise RuntimeError(
             "The session is unavailable because no secret "
             "key was set.  Set the secret_key on the "

src/flask/wrappers.py

@@ -8,6 +8,7 @@ from . import json
 from .globals import current_app
 
 if t.TYPE_CHECKING:
+    import typing_extensions as te
     from werkzeug.routing import Rule
 
 
@@ -91,7 +92,7 @@ class Request(RequestBase):
 
             attach_enctype_error_multidict(self)
 
-    def on_json_loading_failed(self, e: Exception) -> t.NoReturn:
+    def on_json_loading_failed(self, e: Exception) -> "te.NoReturn":
         if current_app and current_app.debug:
             raise BadRequest(f"Failed to decode JSON object: {e}")