Mypy 1.7.0 makes two incorrect inferences about dataclass descriptor-based fields

[jace]

Bug Report

When a dataclass field uses a descriptor, Mypy makes two incorrect inferences:

1. If the descriptor has no default value (it raises AttributeError when accessed on the class), mypy incorrectly assumes it has a default and raises an error on the next field with no default value. The no-default mechanism was documented in Python 3.10 but has been there since 3.7. The specific code is in dataclasses._get_field, as default = getattr(cls, a_name, MISSING).
2. If the descriptor-based field is typed with InitVar, mypy incorrectly assumes the attribute does not exist on the class.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.11&gist=8a47d8538f2fc798a8564d30dae6434e

Expected Behavior

No error, matching runtime behaviour and the internal validations of the @dataclass decorator.

Actual Behavior

Two incorrect errors:

1. Attributes without a default cannot follow attributes with one [misc]
2. "Example2" has no attribute "this_exists" [attr-defined]

Your Environment

• Mypy version used: 1.7.0 and 1.6.1
• Python version used: 3.9 and 3.11

Related: #13856 describes another dataclass descriptor bug, fixed in mypy 1.2.

[jace]

I've found a workaround for the first problem using a TYPE_CHECKING test. Mypy no longer interprets this field as having a default value and will not complain about a subsequent field with no default. This only works if the descriptor's return type is known:

    if TYPE_CHECKING:
        first: str
    else:
        first: FieldDescriptor = FieldDescriptor()

It's murky when the descriptor is a generic or the type is not known. Curiously, Mypy is able to guess the actual type when the field is in a frozen dataclass and accessed from a sub-dataclass if typed like this:

    if TYPE_CHECKING:
        first: Union[FieldDescriptor, Any]
    else:
        first: FieldDescriptor = FieldDescriptor()

1. Revealed type in the instance when it is (a) non-frozen dataclass, (b) non-frozen sub dataclass, (c) frozen dataclass: Union[__main__.FieldDescriptor, Any]

2. Revealed type in the instance when it is (d) a frozen sub dataclass: Union[builtins.str, Any]. Here, the descriptor's type has been correctly replaced with the return type of its __get__ method

3. Revealed type in the class for all four variants is Any

Replacing the Union with one of its two members doesn't work:

• If typed Any, the instance field type is also Any in the frozen sub dataclass, losing the actual type
• If typed FieldDescriptor (the descriptor type), Mypy will report an type error when the dataclass is instantiated

Code: https://mypy-play.net/?mypy=latest&python=3.11&gist=c7e30060be30861c5ec3fac61a5bfb26