Error with TypeVar with constraints on field initialization

[thierry-martinez]

The following example fails to type-check with mypy and pyright, but succeeds with ty:

from dataclasses import dataclass
from typing import TypeVar, Generic

class X:
    pass

class Y(X):
    pass

T = TypeVar("T", X, Y)

@dataclass
class C(Generic[T]):
    v: T

def f(c: C[T]):
    # no error with mypy and ty
    # pyright reports an error on the argument
    c.v = Y()

    # no error with mypy and ty
    # pyright reports an error on the assignment
    d: C[T] = C(Y())

class M(Generic[T]):
    # no error with ty
    # mypy and pyright report an error on the assignment
    c: C[T] = C(Y())

The constraints on T effectively make Y a lower bound for T, meaning that C(Y()) should be considered a valid C[T] for any allowed T. This seems to be correctly understood by mypy for field assignment and constructor assignment within a function, but not for field initialization at the class level.