Make @cached_property generic over the value

[srittau]

Currently the following code will infer type Any for Foo().bar, although it should infer type str:

from werkzeug.utils import cached_property

class Foo:
    @cached_property
    def bar(self) -> str:
        return ""

reveal_type(Foo().bar)

Making class cached_property generic over the value solves this problem and corrects the type of a few fields like Request.base_url. I have a PR ready for this that actually removes a few instances of # type: ignore in werkzeug code (although it adds two new ones.)

[davidism]

I need to see the PR I guess, but note that we can't inherit from typing.Generic until we support only 3.9+ since there's a performance issue with it.

[srittau]

I opened #2117. It's my understanding that any performance issues with Generic should only affect the startup times, unless classes are created on the fly, since cached_property instances are only initiated on startup.

[davidism]

You're right, I wasn't thinking about when cached_property was created, so it should be safe in this case.