Do not handle RoutingExceptions with app error handlers

flask/app.py

@@ -20,7 +20,8 @@ from threading import Lock
 from werkzeug.datastructures import Headers, ImmutableDict
 from werkzeug.exceptions import BadRequest, BadRequestKeyError, HTTPException, \
     InternalServerError, MethodNotAllowed, default_exceptions
-from werkzeug.routing import BuildError, Map, RequestRedirect, Rule
+from werkzeug.routing import BuildError, Map, RequestRedirect, \
+    RoutingException, Rule
 
 from . import cli, json
 from ._compat import integer_types, reraise, string_types, text_type
@@ -1638,6 +1639,12 @@ class Flask(_PackageBoundObject):
         if e.code is None:
             return e
 
+        # RoutingExceptions such as RequestRedirects are special exceptions
+        # used to trigger routing actions such as redirects, and also behave
+        # like proxy exceptions. We return these unchanged as well.
+        if isinstance(e, RoutingException):
+            return e
+
         handler = self._find_error_handler(e)
         if handler is None:
             return e

tests/test_user_error_handler.py

@@ -184,6 +184,10 @@ def test_default_error_handler():
     def forbidden():
         raise Forbidden()
 
+    @app.route('/slash/')
+    def slash():
+        return ''
+
     app.register_blueprint(bp, url_prefix='/bp')
 
     c = app.test_client()
@@ -191,5 +195,6 @@ def test_default_error_handler():
     assert c.get('/bp/forbidden').data == b'bp-forbidden'
     assert c.get('/undefined').data == b'default'
     assert c.get('/forbidden').data == b'forbidden'
+    assert c.get('/slash').location.endswith('/slash/')