Incorrect argument names for dataclass with "private" (mangled) members

Bug Report

Mypy infers dataclass arguments incorrectly when a class has private (__name) members.

To Reproduce

from dataclasses import dataclass


@dataclass(kw_only=True)
class Data:
    __mangled: int


Data(_Data__mangled=1)  # Runs correctly, but mypy complains
Data(__mangled=1)       # Fails, but mypy approves

https://mypy-play.net/?mypy=latest&python=3.13&gist=f585904f6f5652ee86cd11d6502de13f

Expected Behavior

I expect mypy's behaviour to match the execution and requrie the _Data__mangled argument.
Additionally, I expect it to reject the __mangled argument name as unexpected.

Actual Behavior

main.py:9: error: Unexpected keyword argument "_Data__mangled" for "Data"; did you mean "__mangled"? [call-arg]
Found 1 error in 1 file (checked 1 source file)

Your Environment

I reproduced this on mypy-play.net with Python 3.13

• Mypy version used: 1.17.1
• Mypy command-line flags: default mypy-play.net settings
• Python version used: Python 3.13

Inizia con il riproduttore in main.py ed esegui mypy 1.17.1 con Python 3.13, confrontando i nomi delle keyword accettati con il comportamento delle dataclass a runtime. Traccia l’inferenza degli argomenti della dataclass utilizzata per il membro privato e verifica che _Data__mangled sia accettato mentre __mangled sia rifiutato.