Missing Liskov error for subclass with subclass field

[NoamNol]

Mypy is ok with the following code. why?

class Animal():
    pass

class Bird(Animal):
    def fly(self):
        print("fly")

class Farm():
    animal = Animal()

class FarmWithBirds(Farm):
    animal = Bird() # why mypy is ok?

def func(f: Farm):
    f.animal = Animal() # the magic

f = FarmWithBirds()
f.animal.fly() # print "fly"
func(f)
f.animal.fly() # AttributeError: 'Animal' object has no attribute 'fly'

Related: #2984, Incompatible overrides

[intgr]

Note that this works the way I expected when assigning the animal attribute in __init__:

class Animal():
    pass

class Bird(Animal):
    def fly(self):
        print("fly")

class Farm():
    def __init__(self):   # <-- added
        self.animal = Animal()

class FarmWithBirds(Farm):
    def __init__(self):   # <-- added
        self.animal = Bird()

def func(f: Farm):
    f.animal = Animal() # the magic

f = FarmWithBirds()
reveal_type(f.animal) # note: Revealed type is "issue11720.Animal"
f.animal.fly() # error: "Animal" has no attribute "fly"

Not sure why class variables are handled differently in this case.

[AlexWaygood]

Mypy also does not emit any errors for this variation:

from typing import ClassVar

class Animal:
    pass

class Bird(Animal):
    def fly(self) -> None:
        print("fly")

class Farm:
    animal: ClassVar[Animal] = Animal()

class FarmWithBirds(Farm):
    animal: ClassVar[Bird] = Bird() # why mypy is ok?

def func(f: type[Farm]) -> None:
    f.animal = Animal() # the magic

f = FarmWithBirds
f.animal.fly() # print "fly"
func(f)
f.animal.fly() # AttributeError: 'Animal' object has no attribute 'fly'

I think this is a bug.

[hauntsaninja]

Duplicate of #3208