Incompatible type; expected "Optional[T]" with generic dataclasses inheritance

[MarcoRuzzon]

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:

T = TypeVar('T')

@dataclasses.dataclass
class A(Generic[T]):
    attr: typing.Optional[T] = None

A(attr=0)  # OK!

@dataclasses.dataclass
class B(A[int]):
  ...

B(attr=0)
# Argument "attr" to "B" has incompatible type "int"; expected "Optional[T]"

• What is the actual behavior/output?

error: Argument "attr" to "B" has incompatible type "int"; expected "Optional[T]"

• What is the behavior/output you expect?

The code should type check, as it does without dataclass

T = TypeVar('T')

class A(Generic[T]):
    def __init__(self, attr: typing.Optional[T] = None):
        self.attr = attr

A(attr=0)  # OK!

class B(A[int]):
    ...

B(attr=0)  # OK!

• What are the versions of mypy and Python you are using?

mypy 0.910

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

The defaults, executed with: mypy test_script.py

[AlexWaygood]

Closing as a duplicate of #12063