diff --git a/stdlib/2/__builtin__.pyi b/stdlib/2/__builtin__.pyi
index 2e9ea8591b80..9feda83891ae 100644
--- a/stdlib/2/__builtin__.pyi
+++ b/stdlib/2/__builtin__.pyi
@@ -529,7 +529,7 @@ 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: ...
@@ -571,8 +571,8 @@ 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: ...
diff --git a/stdlib/2/builtins.pyi b/stdlib/2/builtins.pyi
index 2e9ea8591b80..9feda83891ae 100644
--- a/stdlib/2/builtins.pyi
+++ b/stdlib/2/builtins.pyi
@@ -529,7 +529,7 @@ 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: ...
@@ -571,8 +571,8 @@ 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: ...
diff --git a/stdlib/3/builtins.pyi b/stdlib/3/builtins.pyi
index 78c598592c77..efd1b5d287c7 100644
--- a/stdlib/3/builtins.pyi
+++ b/stdlib/3/builtins.pyi
@@ -569,7 +569,7 @@ 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: ...
@@ -616,8 +616,8 @@ 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]: ...