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[ty] Temporary hack to reduce false positives around builtins.open() (#20367)

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## Summary

https://github.com/astral-sh/ruff/pull/20165 added a lot of false
positives around calls to `builtins.open()`, because our missing support
for PEP-613 type aliases means that we don't understand typeshed's
overloads for `builtins.open()` at all yet, and therefore always select
the first overload. This didn't use to matter very much, but now that we
have a much stricter implementation of protocol assignability/subtyping
it matters a lot, because most of the stdlib functions dealing with I/O
(`pickle`, `marshal`, `io`, `json`, etc.) are annotated in typeshed as
taking in protocols of some kind.

In lieu of full PEP-613 support, which is blocked on various things and
might not land in time for our next alpha release, this PR adds some
temporary special-casing for `builtins.open()` to avoid the false
positives. We just infer `Todo` for anything that isn't meant to match
typeshed's first `open()` overload. This should be easy to rip out again
once we have proper support for PEP-613 type aliases, which hopefully
should be pretty soon!

## Test Plan

Added an mdtest
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Alex Waygood 2025-09-12 22:20:38 +01:00 • committed by GitHub
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19
crates/ty_python_semantic/resources/mdtest/call/builtins.md
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@ -162,3 +162,22 @@ def _(x: A | B, y: list[int]):
reveal_type(x) # revealed: B & ~A
reveal_type(isinstance(x, B)) # revealed: Literal[True]
```
## Calls to `open()`
We do not fully understand typeshed's overloads for `open()` yet, due to missing support for PEP-613
type aliases. However, we also do not emit false-positive diagnostics on common calls to `open()`:
```py
import pickle
reveal_type(open("")) # revealed: TextIOWrapper[_WrappedBuffer]
reveal_type(open("", "r")) # revealed: TextIOWrapper[_WrappedBuffer]
reveal_type(open("", "rb")) # revealed: @Todo(`builtins.open` return type)
with open("foo.pickle", "rb") as f:
x = pickle.load(f) # fine
def _(mode: str):
reveal_type(open("", mode)) # revealed: @Todo(`builtins.open` return type)
```