add errorhandler type check tests

src/flask/blueprints.py

@@ -561,12 +561,14 @@ class Blueprint(Scaffold):
         )
         return f
 
-    def app_errorhandler(self, code: t.Union[t.Type[Exception], int]) -> t.Callable:
+    def app_errorhandler(
+        self, code: t.Union[t.Type[Exception], int]
+    ) -> t.Callable[[ft.ErrorHandlerDecorator], ft.ErrorHandlerDecorator]:
         """Like :meth:`Flask.errorhandler` but for a blueprint.  This
         handler is used for all requests, even if outside of the blueprint.
         """
 
-        def decorator(f: ft.ErrorHandlerCallable) -> ft.ErrorHandlerCallable:
+        def decorator(f: ft.ErrorHandlerDecorator) -> ft.ErrorHandlerDecorator:
             self.record_once(lambda s: s.app.errorhandler(code)(f))
             return f
 

src/flask/scaffold.py

@@ -654,7 +654,7 @@ class Scaffold:
     @setupmethod
     def errorhandler(
         self, code_or_exception: t.Union[t.Type[Exception], int]
-    ) -> t.Callable[[ft.ErrorHandlerCallable], ft.ErrorHandlerCallable]:
+    ) -> t.Callable[[ft.ErrorHandlerDecorator], ft.ErrorHandlerDecorator]:
         """Register a function to handle errors by code or exception class.
 
         A decorator that is used to register a function given an
@@ -684,7 +684,7 @@ class Scaffold:
                                   an arbitrary exception
         """
 
-        def decorator(f: ft.ErrorHandlerCallable) -> ft.ErrorHandlerCallable:
+        def decorator(f: ft.ErrorHandlerDecorator) -> ft.ErrorHandlerDecorator:
             self.register_error_handler(code_or_exception, f)
             return f
 

src/flask/typing.py

@@ -52,6 +52,7 @@ URLValuePreprocessorCallable = t.Callable[[t.Optional[str], t.Optional[dict]], N
 # https://github.com/pallets/flask/issues/4295
 # https://github.com/pallets/flask/issues/4297
 ErrorHandlerCallable = t.Callable[[t.Any], ResponseReturnValue]
+ErrorHandlerDecorator = t.TypeVar("ErrorHandlerDecorator", bound=ErrorHandlerCallable)
 
 ViewCallable = t.Callable[..., ResponseReturnValue]
 RouteDecorator = t.TypeVar("RouteDecorator", bound=ViewCallable)

tests/typing/typing_error_handler.py

@@ -0,0 +1,33 @@
+from __future__ import annotations
+
+from http import HTTPStatus
+
+from werkzeug.exceptions import BadRequest
+from werkzeug.exceptions import NotFound
+
+from flask import Flask
+
+app = Flask(__name__)
+
+
+@app.errorhandler(400)
+@app.errorhandler(HTTPStatus.BAD_REQUEST)
+@app.errorhandler(BadRequest)
+def handle_400(e: BadRequest) -> str:
+    return ""
+
+
+@app.errorhandler(ValueError)
+def handle_custom(e: ValueError) -> str:
+    return ""
+
+
+@app.errorhandler(ValueError)
+def handle_accept_base(e: Exception) -> str:
+    return ""
+
+
+@app.errorhandler(BadRequest)
+@app.errorhandler(404)
+def handle_multiple(e: BadRequest | NotFound) -> str:
+    return ""