`@dataclass` does not handle `__hash__: None` special case

While working on https://github.com/python/mypy/issues/11463 I found that adding __hash__: None annotations does not work correctly.

Mypy says:

from dataclasses import dataclass

@dataclass(unsafe_hash=False, eq=True, frozen=False)
class SecondCase2:
    x: int
    __hash__: None  # E: Incompatible types in assignment (expression has type "None", base class "object" defined the type as "Callable[[object], int]")
print(SecondCase2(1).__hash__)  # N: Revealed type is "None" \
                                                      # E: Missing positional argument "__hash__" in call to "SecondCase2"

It looks like __hash__ is added to fields and __init__. While runtime version works just fine:

» python out/ex.py
SecondCase2(x=1, __hash__=<slot wrapper '__hash__' of 'object' objects>)

I will fix it in the same PR as https://github.com/python/mypy/issues/11463

Inizia con il reproducer di dataclass fornito e confronta i diagnostic di mypy con il comportamento mostrato a runtime. Traccia il modo in cui i campi di dataclass e init gestiscono hash: None, quindi aggiungi una coverage che confermi che non viene trattato né come campo né come argomento del costruttore; controlla prima l'issue correlata #11463 perché il report indica che questo fix verrà incluso nello stesso PR.