List[Foo] + List[Bar] now returns List[Foo | Bar] (#8293)

Co-authored-by: Alex Waygood <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 3 people

stdlib/builtins.pyi

@@ -986,8 +986,12 @@ class list(MutableSequence[_T], Generic[_T]):
     @overload
     def __setitem__(self, __s: slice, __o: Iterable[_T]) -> None: ...
     def __delitem__(self, __i: SupportsIndex | slice) -> None: ...
+    # Overloading looks unnecessary, but is needed to work around complex mypy problems
+    @overload
     def __add__(self, __x: list[_T]) -> list[_T]: ...
-    def __iadd__(self: Self, __x: Iterable[_T]) -> Self: ...
+    @overload
+    def __add__(self, __x: list[_S]) -> list[_S | _T]: ...
+    def __iadd__(self: Self, __x: Iterable[_T]) -> Self: ...  # type: ignore[misc]
     def __mul__(self, __n: SupportsIndex) -> list[_T]: ...
     def __rmul__(self, __n: SupportsIndex) -> list[_T]: ...
     def __imul__(self: Self, __n: SupportsIndex) -> Self: ...

test_cases/stdlib/builtins/test_list.py

@@ -0,0 +1,19 @@
+from typing import List, Union
+from typing_extensions import assert_type
+
+
+# list.__add__ example from #8292
+class Foo:
+    def asd(self) -> int:
+        return 1
+
+
+class Bar:
+    def asd(self) -> int:
+        return 2
+
+
+combined = [Foo()] + [Bar()]
+assert_type(combined, List[Union[Foo, Bar]])
+for item in combined:
+    assert_type(item.asd(), int)