Missing error when an attribute is a variant generic

The following produces no errors but I expected it to be treated the same as if B.a was a get/set property (which causes a variance error)

class A:
    pass

class A2(A):
    something: str = "hello"
    
T_co = TypeVar('T_co', covariant=True)

class B(Generic[T_co]):
    def __init__(self, a: T_co) -> None:
        self.a: T_co = a  # EXPECTED: typing error here like "public attributes must have invariant type"
        
def set_a(b: B[A]) -> None:
    b.a = A()
    

b2: B[A2] = B(A2())

set_a(b2)

print(b2.a.something) # ACTUAL: runtime error here

Python 3.10, mypy 0.961 with --strict

Start by running the supplied Python 3.10 example with mypy --strict and tracing the variance check for self.a in the generic class B. Compare it with the existing get/set-property handling; done means the invalid covariant mutable attribute is diagnosed and the regression is covered.