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fix list.__add__ to use the expected type #14756

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fix list.__add__ to use the expected type #14756

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9 changes: 9 additions & 0 deletions stdlib/@tests/test_cases/builtins/check_list.py
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Original file line number Diff line number Diff line change
Expand Up @@ -19,3 +19,12 @@ def asd(self) -> int:
assert_type(combined, List[Union[Foo, Bar]])
for item in combined:
assert_type(item.asd(), int)

l_int = [1, 2]
l_str = ["a", "b"]
# TODO: these pass pyright with the current stubs, but mypy erroneously emits errors:
# _1: List[object] = l_int + l_str
# _2: List[None] = l_int + l_str

# combined = l_int + l_str
# assert_type(combined, List[int | str])
7 changes: 2 additions & 5 deletions stdlib/builtins.pyi
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Expand Up @@ -1135,11 +1135,8 @@ class list(MutableSequence[_T]):
@overload
def __setitem__(self, key: slice, value: Iterable[_T], /) -> None: ...
def __delitem__(self, key: SupportsIndex | slice, /) -> None: ...
# Overloading looks unnecessary, but is needed to work around complex mypy problems
@overload
def __add__(self, value: list[_T], /) -> list[_T]: ...
@overload
def __add__(self, value: list[_S], /) -> list[_S | _T]: ...
# `__add__` returns a new object, so we capture the expected result type with a type variable
def __add__(self, value: list[_S], /) -> list[_T1 | _T | _S]: ...
def __iadd__(self, value: Iterable[_T], /) -> Self: ... # type: ignore[misc]
def __mul__(self, value: SupportsIndex, /) -> list[_T]: ...
def __rmul__(self, value: SupportsIndex, /) -> list[_T]: ...
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