It's unsound to subclass an order=True dataclass, because the generated comparison methods raise an exception if you try to compare an instance of the subclass with an instance of the superclass:

from dataclasses import dataclass

@dataclass(order=True)
class F:
    x: int

class G(F): ...

G(42) < F(42)  # exception raised here at runtime, but cannot detected by ty

We should detect attempts to subclass order=True dataclasses and warn users about the unsoundness (emitting a diagnostic at the point where G is defined). We could recommend that they use functools.total_ordering instead to generate their comparison methods, as total_ordering doesn't have the same problem.

(One way of describing the problem here is that the design of the stdlib feature violates the Liskov Substitution Principle.)