Final TypedDict

[JukkaL]

We could perhaps support final TypedDicts that don't allow subtypes to have extra keys. Example:

from typing import final, TypedDict

@final
class A(TypedDict):
    x: int

@final
class B(TypedDict):
    x: int
    y: str

def f(d: A) -> None: ...

d: B = {...}
f(d)  # Error -- extra key 'y'

Here are some random thoughts:

• We should be able to infer a more precise type for values(), since there can't be extra "hidden" values with arbitrary types.
• A non-final TypedDict isn't compatible with a final one, since it could have some extra keys due to structural subtyping.
• This could help with exhaustiveness checking in match statements.

I haven't thought carefully about all the implications of this feature. Before implementing this, some additional analysis would be useful.

See #10759 for more context.

[97littleleaf11]

Previous discussion: #7981

[JukkaL]

Closing as dupe of #7981, oops.