fix issue4297

CHANGES.rst

@@ -24,6 +24,8 @@ Released 2021-10-04
 -   Fix the order of ``before_request`` and other callbacks that trigger
     before the view returns. They are called from the app down to the
     closest nested blueprint. :issue:`4229`
+-   Fix ``Blueprint.errorhandler`` restricts type of result function which
+    avoids chaining. :issue:`4297`
 
 
 Version 2.0.1

src/flask/scaffold.py

@@ -80,7 +80,7 @@ class Scaffold:
     name: str
     _static_folder: t.Optional[str] = None
     _static_url_path: t.Optional[str] = None
-
+    T = t.TypeVar("T", t.Callable[..., t.Any], t.Any)
     #: JSON encoder class used by :func:`flask.json.dumps`. If a
     #: blueprint sets this, it will be used instead of the app's value.
     json_encoder: t.Optional[t.Type[JSONEncoder]] = None
@@ -646,12 +646,7 @@ class Scaffold:
         return f
 
     @setupmethod
-    def errorhandler(
+    def errorhandler(self, code_or_exception: t.Union[T, int]) -> T:
-        self, code_or_exception: t.Union[t.Type[GenericException], int]
-    ) -> t.Callable[
-        ["ErrorHandlerCallable[GenericException]"],
-        "ErrorHandlerCallable[GenericException]",
-    ]:
         """Register a function to handle errors by code or exception class.
 
         A decorator that is used to register a function given an