Allow method assignment in body of type extending typing.NamedTuple

this currently works for non-namedtuple classes (both runtime and mypy-time):

class C:
    x: int

    __hash__ = object.__hash__

and it works for typing.NamedTuple at runtime:

from typing import NamedTuple

class C(NamedTuple):
    x: int

    __hash__ = object.__hash__

$ python3 -i t2.py
>>> c = C(1)
>>> hash(c)
-9223363269847169460
>>> object.__hash__(c)
-9223363269847169460
>>> tuple.__hash__(c)
3430019387558

but fails at type-checking time:

$ mypy --version && python --version --version
mypy 0.770
Python 3.6.8 (default, Oct  7 2019, 12:59:55) 
[GCC 8.3.0]
$ mypy t2.py
t2.py:7: error: NamedTuple field name cannot start with an underscore: __hash__
t2.py:7: error: Invalid statement in NamedTuple definition; expected "field_name: field_type [= default]"
Found 2 errors in 1 file (checked 1 source file)

concrete usecase: __hash__ is in the critical path (~13%) for some code I'm writing and the extra layer of function which calls into object.__hash__'s overhead is significant:

# mypy is ok with this
from typing import NamedTuple

class C(NamedTuple):
    x: int

    def __hash__(self) -> int:
        return object.__hash__(self)

I'd be happy to hack on this given some pointers :)

Reproduisez le problème avec les exemples de t2.py et exécutez mypy sur le fichier afin de confirmer les deux diagnostics pour hash. Suivez la manière dont les corps de classe NamedTuple distinguent les champs des affectations de méthodes, puis faites en sorte que l’affectation passe la vérification de types tout en préservant la validation des déclarations de champs invalides.