In python/typeshed#5869, __class_getitem__ was added to the stub for os.PathLike. The method exists on the class at runtime; however, the addition of the method caused regressions in mypy 0.920. PathLike is defined in typeshed as a protocol, and so the addition of the method to the stub led mypy to erroneously believe that a custom class could only be considered a subtype of PathLike if it implemented __class_getitem__. The change to the stub had to be reverted in python/typeshed#6591, in time for mypy 0.930.

The question is: is there a situation where it is ever useful for __class_getitem__ to be considered part of a protocol interface? Or, would it be better for __class_getitem__ to be special-cased, such that mypy does not consider it to be part of the interface, even if it is defined on a Protocol class?

Related: #11884, where a similar issue is being discussed w.r.t. __slots__. Cc. @sobolevn, who is working on the __slots__ issue, and @hauntsaninja, who filed the PR to fix the PathLike regression.