Mutable attributes of classes should be invariant in subclasses.

[mikeshardmind]

Pyre Bug

Bug description

Subclasses should not be able to change the type of their mutable attributes, see relevant open issue in mypy and recent discussion in python/typing

Reproduction steps

class UStr(str):
    pass


class A:
    foo: str
    def __init__(self):
        self.foo = ""

class B(A):
    foo: UStr
    def __init__(self):
        self.foo = UStr("")

Expected behavior
Pyre should not allow the subclass B

Logs

No Errors!

Please run your reproduction steps followed by pyre rage > pyre_rage.log, and upload the file here:

Additional context
This was run in the playground to verify, as I am not a pyre user, only bringing this to the attention of pyre due to ecosystem impact

[connernilsen]

Hey @mikeshardmind, thanks for reporting this to us! I think this is something that we have run into trouble with before, but we didn't have the capacity at the time to implement a fix. I'll go through and see if it's something we're interested in coming back to, but either way, I'll add it to our backlog.