fix list.__add__ to use the expected type

Author: KotlinIsland

stdlib/@tests/test_cases/builtins/check_list.py

@@ -19,3 +19,23 @@ def asd(self) -> int:
 assert_type(combined, List[Union[Foo, Bar]])
 for item in combined:
     assert_type(item.asd(), int)
+
+# ignoring this so we can test mypy and pyright separately
+# pyright: reportUnnecessaryTypeIgnoreComment=false
+
+l_int = [1, 2]
+l_str = ["a", "b"]
+combined1 = l_int + l_str
+assert_type(combined1, List[int | str])
+
+combined2: list[str | int]
+# mypy doesn't support this case
+combined2 = l_int + l_str  # type: ignore[operator]
+assert_type(combined2, List[str | int])
+
+combined2 = list[str]() + list[int | str]()
+assert_type(combined2, List[str | int])
+
+# mypy doesn't support this case
+combined3: list[object] = l_int + l_str  # type: ignore[operator]
+assert_type(combined3, List[object])

stdlib/builtins.pyi

@@ -1137,9 +1137,10 @@ class list(MutableSequence[_T]):
     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]: ...
+    # # `__add__` returns a new object, so we capture the expected result type with a type variable
+    def __add__(self, value: list[_T], /) -> list[_T1 | _T]: ...
     @overload
-    def __add__(self, value: list[_S], /) -> list[_S | _T]: ...
+    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]: ...