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[red-knot] Fix edge case for binary-expression inference where the lhs and rhs are the exact same type (#13823)

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## Summary

This fixes an edge case that @carljm and I missed when implementing
https://github.com/astral-sh/ruff/pull/13800. Namely, if the left-hand
operand is the _exact same type_ as the right-hand operand, the
reflected dunder on the right-hand operand is never tried:

```pycon
>>> class Foo:
...     def __radd__(self, other):
...         return 42
...         
>>> Foo() + Foo()
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    Foo() + Foo()
    ~~~~~~^~~~~~~
TypeError: unsupported operand type(s) for +: 'Foo' and 'Foo'
```

This edge case _is_ covered in Brett's blog at
https://snarky.ca/unravelling-binary-arithmetic-operations-in-python/,
but I missed it amongst all the other subtleties of this algorithm. The
motivations and history behind it were discussed in
https://mail.python.org/archives/list/python-dev@python.org/thread/7NZUCODEAPQFMRFXYRMGJXDSIS3WJYIV/

## Test Plan

I added an mdtest for this cornercase.
This commit is contained in:
Alex Waygood 2024-10-19 19:09:54 +01:00 • committed by GitHub
parent f4b5e70fae
commit 55bccf6680
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2 changed files with 27 additions and 4 deletions
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12
crates/red_knot_python_semantic/src/types/infer.rs
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@ -2682,10 +2682,14 @@ impl<'db> TypeInferenceBuilder<'db> {
.call(self.db, &[left_ty, right_ty])
.return_ty(self.db)
.or_else(|| {
right_class
.class_member(self.db, op.reflected_dunder())
.call(self.db, &[right_ty, left_ty])
.return_ty(self.db)
if left_class == right_class {
None
} else {
right_class
.class_member(self.db, op.reflected_dunder())
.call(self.db, &[right_ty, left_ty])
.return_ty(self.db)
}
})
}