Incorrect type inference after `__setitem__` / `__getitem__`

[MarcoGorelli]

Describe the Bug

from __future__ import annotations

from typing import Any, reveal_type, Protocol

class Series(Protocol):
    def astype(self, dtype: Any) -> Series:
        ...

class DataFrame(Protocol):
    def __setitem__(self, key: str, value: object) -> None:
        ...

    def __getitem__(self, key: str) -> Series:
        ...

def main(df: DataFrame) -> None:
    df['a'] = '!'
    df['a'] = df['a'].astype('string[pyarrow')

(scratch) mgorelli@marcoslaptop:~/scratch$ mypy t.py
Success: no issues found in 1 source file
(scratch) mgorelli@marcoslaptop:~/scratch$ ty check t.py
All checks passed!
(scratch) mgorelli@marcoslaptop:~/scratch$ pyright t.py
0 errors, 0 warnings, 0 informations
(scratch) mgorelli@marcoslaptop:~/scratch$ zuban check t.py
Success: no issues found in 1 source file
(scratch) mgorelli@marcoslaptop:~/scratch$ pyrefly check t.py
ERROR Object of class `str` has no attribute `astype` [missing-attribute]
  --> t.py:18:15
   |
18 |     df['a'] = df['a'].astype('string[pyarrow')
   |               ^^^^^^^^^^^^^^
   |
 INFO 1 error

Sandbox Link

https://pyrefly.org/sandbox/?project=N4IgZglgNgpgziAXKOBDAdgEwEYHsAeAdAA4CeS4ATrgLYAEA%2Bg2AK4AuLlMTdENxuSmzoZ0uNqjYRc6OAB10CsNXptSxCOgDmvfoOEBBdKQA0dLgDcYqKAzXEYZgArU2uAMa4oChe6io4ODoAZRhKCHgAChdxDy8ASkQFOhS6TBgwETh7GEi4GCgwM0wcxDojUni6AFoAPhCwiLgk9FS2ukJOn3Q-AKCAEUlUADFKVBpcmLdPKETk1PTMpnypNhgaJjyCoroAaxhSMuzKMwsbFhgy3GwAKxh3Niq6ugA5GUv59o6uxVaFjMYDC0MFW602%2BUKZn2hzoxye9VC4XgLS%2BKU6hG6izoNFQmkimDAZUGElG4xg8Ne7xRCzAAG0AOSoekAXToAF46PSAIT0z5pOmMlns-kMpnMwgBHKRenHTRaWlkVCUagAd3p8QUIBMIDIXDAUFIhDcNCgFAAxHQnKQ9QbYRgcAQ6J50JAtJxJNJ0Bj0BbQjA6AALNhsYjNAD0Yd1GQNhEEWjDMHQYcwHjgYedrvdUhkYboYEEIjO0FQ2FgTpkmbG2dauGI1bg3rIbADMmqVkocE9wrkIAAzIQAIwAJh7ClpYWoHeZChY6D4AiEMEw1UwEC4DwgVmF9IgWjEXF56DEbGqXAAjiw10vqtDqqh3O54EEOfSVUr0IeQABfbX3qRWYZoBgCg0CwPAiDIb8gA

(Only applicable for extension issues) IDE Information

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