Frozen dataclasses should have an implicit Final annotation on its members

[alecov]

Bug Report

Frozen dataclasses should be implicitly Final on its members.

Currently mypy detects correctly that a frozen dataclass member cannot be assigned to; this is similar (if not altogether equivalent) to having a Final annotation on that (and every) member. This is important because it affects type covariance:

To Reproduce

from typing import cast
from dataclasses import dataclass

class C: pass
class D(C): pass

@dataclass(frozen=True)
class MyData[T: C]:
    m: T

def f(x: MyData[C]) -> None: pass

f(cast(MyData[D], 0)) # error, T is invariant here because `m` is public and deemed assignable

However, since MyData above is frozen, m cannot be assigned to and can be deemed final for purposes of covariance.

Expected Behavior

The above should typecheck:

f(cast(MyData[D], 0)) # this should typecheck

Actual Behavior

The above doesn't typecheck.

Your Environment

• Mypy version used: 1.18.2
• Mypy command-line flags: nothing special
• Mypy configuration options from mypy.ini (and other config files): strict mode = True
• Python version used: 3.13.7

[alecov]

The current workaround is to use an explicit TypeVar with covariance=True in cases where covariance of this kind is desirable. There are no consequences other than having uglier code. Also, note that currently there is no way to explicitly annotate members as Final, because mypy does not accept typevars as arguments to Final.

[erictraut]

This issue is discussed here. The invariance is due to the synthesized __replace__ method, which was introduced in Python 3.13.

[alecov]

Good, I think we can close this as a duplicate, then.