well it's runtime because you run the program to see it happen

this is actually a curious use case that I hadn't considered, which is that the base class here isn't actually a dataclass since it's your base. This implies things like mixins would work too which I also never considered, like this:

class Parent(MappedAsDataclass, DeclarativeBase):
    pass


class Sub(Parent):
    __abstract__ = True
    a: InitVar[int] = field()

class Child(Sub):
    __tablename__ = "child"

    b: Mapped[int] = mapped_column(primary_key=True)

so none of this is set up right now, I'd have to look and see how hard it is to make patterns like these work.

like this doesn't work either:

from dataclasses import InitVar, dataclass


class Parent:
    a: InitVar[int]


@dataclass
class Child(Parent):
    __tablename__ = "child"

    b: int


Child(1, 2)

because Parent isn't a dataclass. that's essentially what's happening here

annnnd....it looks like your program works if we just do this:

from dataclasses import InitVar, dataclass
from sqlalchemy.orm import Mapped, mapped_column, DeclarativeBase, MappedAsDataclass


@dataclass
class Parent(MappedAsDataclass, DeclarativeBase):
    a: InitVar[int]


class Child(Parent):
    __tablename__ = "child"
    b: Mapped[int] = mapped_column(primary_key=True)


Child(1, 2)

so....try that as a workaround and see if there are any issues? We would essentially look into making all classes into dataclasses, not just the mapped ones.

NOTE - the change we hope to make in 2.0.1 will remove the need for the above workaround, but also the above workaround will fail at runtime once we make that change. For people reading this please be cautious