dataclass: Incompatible types in assignment to Optional[List[Optional[int]]]

Bug Report

In dataclasses, using default_factory with an Optional[List[Optional[int]]] field produces an "Incompatible types in assignment" error

To Reproduce

$ python --version
Python 3.9.5

$ mypy --version
mypy 0.902

$ cat tmp.py
from dataclasses import dataclass, field
from typing import Optional, List
@dataclass
class SubscriptedList:
    list: List[int] = field(default_factory=lambda: [1, 2])
    opt_list: Optional[List[int]] = field(default_factory=lambda: [1, 2])
    list_opt: List[Optional[int]] = field(default_factory=lambda: [1, 2])
    opt_list_opt: Optional[List[Optional[int]]] = field(default_factory=lambda: [1, 2])

$ mypy tmp.py
tmp.py:8: error: Incompatible types in assignment (expression has type "List[int]", variable has type "Optional[List[Optional[int]]]")
Found 1 error in 1 file (checked 1 source file)

This error does not come up for a normal variable assignment:

var: Optional[List[Optional[int]]] = [1,2]  # mypy has no issue with this

If the default is [1,2, None] or None, no error is produced, but using [None] as the default does produce an error.

Note that a very similar error presents for Optional[Dict[str, Optional[int]]]:

from dataclasses import dataclass, field
from typing import Dict, Optional
@dataclass
class SubscriptedDict:
    opt_dict_opt: Optional[Dict[str, Optional[int]]] = field(default_factory=lambda: {"foo": 123}) 

With the above, mypy gives:

tmp.py:5: error: Incompatible types in assignment (expression has type "Dict[str, int]", variable has type "Optional[Dict[str, Optional[int]]]")

Expected Behavior

I do not expect that assigning default_factory=lambda: [1,2] to an Optional[List[Optional[int]]] field produces an error.

Your Environment

• Mypy version used: 0.902
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.9.5
• Operating system and version: Ubuntu 18.04

Start by reproducing the report in tmp.py with mypy 0.902, focusing on the dataclass fields using Optional[List[Optional[int]]] and Optional[Dict[str, Optional[int]]]. Compare these results with the normal variable assignment; done means valid default_factory expressions no longer produce an incompatible-assignment error while genuinely incompatible defaults remain errors.