Inheriting from a frozen class causes instantiated child hashes to collide

This works as expected:

class A:
    pass

class B(A):
    pass

class C(A):
    pass

assert hash(B) != hash(C)
assert hash(B()) != hash(C())

=========================== 1 passed in 3.39 seconds ===========================

This errors:

import attr

@attr.s(frozen=True)
class A:
    pass

class B(A):
    pass

class C(A):
    pass

assert hash(B) != hash(C)
assert hash(B()) != hash(C())

>       assert hash(B()) != hash(C())
E       AssertionError: assert 7618298384354620882 != 7618298384354620882
E        +  where 7618298384354620882 = hash(B())
E        +    where B() = <class 'test_attr_bug.<locals>.B'>()
E        +  and   7618298384354620882 = hash(C())
E        +    where C() = <class 'test_attr_bug.<locals>.C'>()

So these two instantiated classes hash to the same value despite a) being instances of different classes and b) being of classes which, when uninstantiated, have differing hashes.

This seems very wrong to me!

Start by running the Python examples from the issue and confirm that instances of the two frozen subclasses produce the same hash. Then trace attrs' generated hashing for frozen classes and add a regression test covering distinct subclasses; done means the instantiated B and C hashes differ without breaking the existing frozen-class behavior.