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[red-knot] Fix edge case for binary-expression inference where the lh…
…s and rhs are the exact same type (#13823) ## Summary This fixes an edge case that @carljm and I missed when implementing #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.
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