Case-insensitivity of keys doesn't extend to set operations

[Dreamsorcerer]

Would be nice to have this work correctly:

>>> d = multidict.CIMultiDict()
>>> d["CONTENT-LENGTH"] = "4"
>>> "Content-Length" in d.keys()
True
>>> {"Content-Length"} & d.keys()
set()  # Should get a result here
>>> {"CONTENT-LENGTH"} & d.keys()
{'CONTENT-LENGTH'}

I tried to do this in aiohttp to check if any of a set of headers is present.

[asvetlov]

it seems that the feature request requires case insensitive set implemented first.
In turn, it raises additional questions:
What is the type of CISet() & set()? Should it be ci set or bare set? The same for set() & CISet()?

One possible solution could be returning CI Set for all binary operations if one of two arguments is also CI Set.

[asvetlov]

It seems that we cannot guarantee that CI Set is always returned; the type of a & b is type(a) for sets at least for their ABCs:

>>> frozenset(['a']) & {'b'}
frozenset()
>>> {'b'} & frozenset(['a'])
set()
>>> 

[Dreamsorcerer]

I think that can be changed by using __rand__, can't it?

But, I don't think it's necessary to have a CISet here, the main thing is just to ensure that the keys are returned. So, adding __add__/__or__ to KeysView and returning a set which contains the items in other which case-insensitively match keys in the KeysView should be sufficient.