Correctly type `vary_on_headers` decorator

[intgr]

What's wrong

Noticed this while reviewing #1339. The vary_on_headers decorator doesn't preserve the type (arguments and return) when used on a function.

[RyanWalker277]

Hey @intgr @adamchainz! Wanted to ask a few things about this one. In https://github.com/typeddjango/django-stubs/blob/master/django-stubs/views/decorators/vary.pyi , what does the definition of vary_on_headers here imply? Are we overriding the decorator funciton here? One approach I could think of is wrapping the whole definition into functools.wraps decorator. Not sure how good that would be or whether it would even work or not? Would appreciate your feedback!

[intgr]

functools.wraps doesn't help AFAIK. I could be wrong.

There are lots of examples of decorators already, I think you can look at require_http_methods which behaves similarly.

Here's mypy documentation on type hinting decorators: https://mypy.readthedocs.io/en/stable/generics.html#declaring-decorators

[adamchainz]

Please don't @ particular people on issues, those who have notifications will notice.

[RyanWalker277]

I have raised a PR ( #1370 ) for this. For a more accurate type annotation, I utilised a parameter specification (ParamSpec). Kindly review.