`Cannot inherit frozen dataclass from a non-frozen one` error when using pydantic `frozen=True`

[johnnyleitrim]

Bug Report

Specifying frozen=True as model class kwargs for pydantic models causes the following error to be emitted from mypy:

Cannot inherit frozen dataclass from a non-frozen one

To Reproduce

1. Install pydantic and mypy: pip install pydantic mypy
2. Create test.py with the following content:

from pydantic import BaseModel

class MyModel(BaseModel, frozen=True):
    my_val: int

3. Run mypy on the test file: mypy test.py

Expected Behavior
No errors reported (as was the case with mypy 1.0.1):

> mypy test.py
Success: no issues found in 1 source file

Actual Behavior
Errors reported:

> mypy test.py
test.py:3: error: Cannot inherit frozen dataclass from a non-frozen one  [misc]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.1.1
• Python version used: 3.10

[wesleywright]

Similar to in #14850, I believe this is actually an issue with the Pydantic library's use of dataclass_transform. From looking at the code, it seems that BaseModel is itself using the transform and should be treated as a dataclass, so per the standard, frozen dataclasses cannot subclass it.

You may be able to use the same workaround as suggested in #14850 (using the latest version of the Pydantic plugin).

[wesleywright]

After double checking the spec (whoops), there is actually a provision for this:

Frozen dataclasses cannot inherit from non-frozen dataclasses. A class that has been decorated with dataclass_transform is considered neither frozen nor non-frozen, thus allowing frozen classes to inherit from it. Similarly, a class that directly specifies a metaclass that is decorated with dataclass_transform is considered neither frozen nor non-frozen.

This seems to be the only way to declare a class that has dataclass semantics but which is treated as neither frozen nor non-frozen. This is definitely a bug in MyPy; I will try to create a PR to fix it.