dataclass does not properly support generics

[hackaugusto]

Please provide more information to help us understand the issue:

• Are you reporting a bug, or opening a feature request?

I believe this is a bug.

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

Here is MRE:

from dataclasses import dataclass
from typing import Generic, NewType, TypeVar

T = TypeVar("T")
V = NewType("V", int)

@dataclass
class Base(Generic[T]):
    attr: T

# If this decorator is commented than the generic works as expected
@dataclass
class Sub(Base[V]):
    pass

Sub(attr=V(1))

• What is the actual behavior/output?

Argument "attr" to "Sub" has incompatible type "V"; expected "T"

• What is the behavior/output you expect?

The code should type check.

• What are the versions of mypy and Python you are using?
  Do you see the same issue after installing mypy from Git master?

mypy 0.720
yes, tested against: 8782ae7f789ad5b8cab97d3c8419f9f98b6eb285

• What are the mypy flags you are using? (For example --strict-optional)

The defaults, executed with: mypy test_script.py

• If mypy crashed with a traceback, please paste
  the full traceback below.

[ilevkivskyi]

Yes, this is a bug. The fix is to map/expand variable types in the supertype along the MRO instead of "blindly" copying them to the subtype. Should be not too hard to implement.

[natemcmaster]

It looks like this is still a problem with the latest release. I think I know how to solve this. I can send a pull request.

Also, looks like the same problem affects the attrs plugin (#5744). I think I can fix both with basically the same code.

[natemcmaster]

I opened PRs to fix:

• For dataclasses - Fix generic inheritance for dataclass init methods #9380
• For attrs - Fix generic inheritance for attrs init methods #9383