fix list concatenation (#2404)

Fixes #2383, python/mypy#5492.

Author: JelleZijlstra

stdlib/2/__builtin__.pyi

@@ -547,7 +547,7 @@ class tuple(Sequence[_T_co], Generic[_T_co]):
     def __le__(self, x: Tuple[_T_co, ...]) -> bool: ...
     def __gt__(self, x: Tuple[_T_co, ...]) -> bool: ...
     def __ge__(self, x: Tuple[_T_co, ...]) -> bool: ...
-    def __add__(self, x: Tuple[_T_co, ...]) -> Tuple[_T_co, ...]: ...
+    def __add__(self, x: Tuple[_S, ...]) -> Tuple[Union[_T_co, _S], ...]: ...
     def __mul__(self, n: int) -> Tuple[_T_co, ...]: ...
     def __rmul__(self, n: int) -> Tuple[_T_co, ...]: ...
     def count(self, x: Any) -> int: ...
@@ -589,8 +589,8 @@ class list(MutableSequence[_T], Generic[_T]):
     def __setslice__(self, start: int, stop: int, o: Sequence[_T]) -> None: ...
     def __delitem__(self, i: Union[int, slice]) -> None: ...
     def __delslice__(self, start: int, stop: int) -> None: ...
-    def __add__(self, x: List[_T]) -> List[_T]: ...
-    def __iadd__(self, x: Iterable[_T]) -> List[_T]: ...
+    def __add__(self, x: List[_S]) -> List[Union[_T, _S]]: ...
+    def __iadd__(self, x: Iterable[_T]) -> List[_T]: ...  # type: ignore
     def __mul__(self, n: int) -> List[_T]: ...
     def __rmul__(self, n: int) -> List[_T]: ...
     def __contains__(self, o: object) -> bool: ...

stdlib/2/builtins.pyi

@@ -547,7 +547,7 @@ class tuple(Sequence[_T_co], Generic[_T_co]):
     def __le__(self, x: Tuple[_T_co, ...]) -> bool: ...
     def __gt__(self, x: Tuple[_T_co, ...]) -> bool: ...
     def __ge__(self, x: Tuple[_T_co, ...]) -> bool: ...
-    def __add__(self, x: Tuple[_T_co, ...]) -> Tuple[_T_co, ...]: ...
+    def __add__(self, x: Tuple[_S, ...]) -> Tuple[Union[_T_co, _S], ...]: ...
     def __mul__(self, n: int) -> Tuple[_T_co, ...]: ...
     def __rmul__(self, n: int) -> Tuple[_T_co, ...]: ...
     def count(self, x: Any) -> int: ...
@@ -589,8 +589,8 @@ class list(MutableSequence[_T], Generic[_T]):
     def __setslice__(self, start: int, stop: int, o: Sequence[_T]) -> None: ...
     def __delitem__(self, i: Union[int, slice]) -> None: ...
     def __delslice__(self, start: int, stop: int) -> None: ...
-    def __add__(self, x: List[_T]) -> List[_T]: ...
-    def __iadd__(self, x: Iterable[_T]) -> List[_T]: ...
+    def __add__(self, x: List[_S]) -> List[Union[_T, _S]]: ...
+    def __iadd__(self, x: Iterable[_T]) -> List[_T]: ...  # type: ignore
     def __mul__(self, n: int) -> List[_T]: ...
     def __rmul__(self, n: int) -> List[_T]: ...
     def __contains__(self, o: object) -> bool: ...

stdlib/3/builtins.pyi

@@ -589,7 +589,7 @@ class tuple(Sequence[_T_co], Generic[_T_co]):
     def __le__(self, x: Tuple[_T_co, ...]) -> bool: ...
     def __gt__(self, x: Tuple[_T_co, ...]) -> bool: ...
     def __ge__(self, x: Tuple[_T_co, ...]) -> bool: ...
-    def __add__(self, x: Tuple[_T_co, ...]) -> Tuple[_T_co, ...]: ...
+    def __add__(self, x: Tuple[_S, ...]) -> Tuple[Union[_T_co, _S], ...]: ...
     def __mul__(self, n: int) -> Tuple[_T_co, ...]: ...
     def __rmul__(self, n: int) -> Tuple[_T_co, ...]: ...
     def count(self, x: Any) -> int: ...
@@ -636,8 +636,8 @@ class list(MutableSequence[_T], Generic[_T]):
     @overload
     def __setitem__(self, s: slice, o: Iterable[_T]) -> None: ...
     def __delitem__(self, i: Union[int, slice]) -> None: ...
-    def __add__(self, x: List[_T]) -> List[_T]: ...
-    def __iadd__(self, x: Iterable[_T]) -> List[_T]: ...
+    def __add__(self, x: List[_S]) -> List[Union[_T, _S]]: ...
+    def __iadd__(self, x: Iterable[_T]) -> List[_T]: ...  # type: ignore
     def __mul__(self, n: int) -> List[_T]: ...
     def __rmul__(self, n: int) -> List[_T]: ...
     def __imul__(self, n: int) -> List[_T]: ...