Mypy 1.7.0 makes two incorrect inferences about dataclass descriptor-based fields

Bug Report

When a dataclass field uses a descriptor, Mypy makes two incorrect inferences:

1. If the descriptor has no default value (it raises AttributeError when accessed on the class), mypy incorrectly assumes it has a default and raises an error on the next field with no default value. The no-default mechanism was documented in Python 3.10 but has been there since 3.7. The specific code is in dataclasses._get_field, as default = getattr(cls, a_name, MISSING).
2. If the descriptor-based field is typed with InitVar, mypy incorrectly assumes the attribute does not exist on the class.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.11&gist=8a47d8538f2fc798a8564d30dae6434e

Expected Behavior

No error, matching runtime behaviour and the internal validations of the @dataclass decorator.

Actual Behavior

Two incorrect errors:

1. Attributes without a default cannot follow attributes with one [misc]
2. "Example2" has no attribute "this_exists" [attr-defined]

Your Environment

• Mypy version used: 1.7.0 and 1.6.1
• Python version used: 3.9 and 3.11

Related: #13856 describes another dataclass descriptor bug, fixed in mypy 1.2.

Inizia dalla riproduzione collegata di mypy-play e confronta i suoi due errori segnalati con il comportamento dei descrittori documentato intorno a dataclasses._get_field. Traccia il percorso del controllo dei tipi delle dataclass per i campi descrittore, incluso InitVar, e aggiungi una copertura mirata per entrambi i casi. Il lavoro è completato quando mypy non segnala errori per la riproduzione nelle versioni di Python interessate.