## Summary
Negative subscripts are also indicative of a thing being subcriptable:
```python
class Subscriptable:
def __getitem__(self, key: int) -> int:
return 42
class NotSubscriptable: ...
def _(x: list[Subscriptable | NotSubscriptable]):
if not isinstance(x[-1], NotSubscriptable):
# After narrowing, x[-1] excludes NotSubscriptable, which means subscripting works
reveal_type(x[-1]) # revealed: Subscriptable & ~NotSubscriptable
reveal_type(x[-1][0]) # revealed: int
```
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