Fix type-hint in CallbackDict

CHANGES.rst

@@ -34,6 +34,8 @@ Unreleased
 -   Ad-hoc TLS certs are generated with SAN matching CN. :issue:`2158`
 -   Fix memory usage for locals when using Python 3.6 or pre 0.4.17
     greenlet versions. :pr:`2212`
+-   Fix type anntoation in ``CallbackDict``, because it is not
+    utilizing a bound TypeVar. :issue:`2235`
 
 
 Version 2.0.1

src/werkzeug/datastructures.pyi

@@ -29,6 +29,7 @@ K = TypeVar("K")
 V = TypeVar("V")
 T = TypeVar("T")
 D = TypeVar("D")
+_CD = TypeVar("_CD", bound="CallbackDict")
 
 def is_immutable(self: object) -> NoReturn: ...
 def iter_multi_items(
@@ -685,7 +686,7 @@ class CallbackDict(UpdateDictMixin[K, V], Dict[K, V]):
     def __init__(
         self,
         initial: Optional[Union[Mapping[K, V], Iterable[Tuple[K, V]]]] = None,
-        on_update: Optional[Callable[[CallbackDict], None]] = None,
+        on_update: Optional[Callable[[_CD], None]] = None,
     ) -> None: ...
 
 class HeaderSet(Set[str]):

src/werkzeug/sansio/response.py

@@ -316,7 +316,7 @@ def mimetype_params(self) -> t.Dict[str, str]:
         .. versionadded:: 0.5
         """
 
-        def on_update(d: t.Dict[str, str]) -> None:
+        def on_update(d: CallbackDict) -> None:
             self.headers["Content-Type"] = dump_options_header(self.mimetype, d)
 
         d = parse_options_header(self.headers.get("content-type", ""))[1]

src/werkzeug/test.py

@@ -570,7 +570,7 @@ def mimetype_params(self) -> t.Mapping[str, str]:
         .. versionadded:: 0.14
         """
 
-        def on_update(d: t.Mapping[str, str]) -> None:
+        def on_update(d: CallbackDict) -> None:
             self.headers["Content-Type"] = dump_options_header(self.mimetype, d)
 
         d = parse_options_header(self.headers.get("content-type", ""))[1]