slices of MutableSequences are also MutableSequences (#2428)

stdlib/2/UserString.pyi

@@ -13,9 +13,9 @@ class UserString(Sequence[UserString]):
     def __hash__(self) -> int: ...
     def __len__(self) -> int: ...
     @overload
-    def __getitem__(self: _UST, i: int) -> _UST: ...
+    def __getitem__(self: _UST, i: int) -> _UST: ...  # type: ignore
     @overload
-    def __getitem__(self: _UST, s: slice) -> _UST: ...
+    def __getitem__(self: _UST, s: slice) -> _UST: ...  # type: ignore
     def __add__(self: _UST, other: Any) -> _UST: ...
     def __radd__(self: _UST, other: Any) -> _UST: ...
     def __mul__(self: _UST, other: int) -> _UST: ...
@@ -61,7 +61,11 @@ class UserString(Sequence[UserString]):
     def upper(self: _UST) -> _UST: ...
     def zfill(self: _UST, width: int) -> _UST: ...
 
-class MutableString(UserString, MutableSequence[MutableString]):
+class MutableString(UserString, MutableSequence[MutableString]):  # type: ignore
+    @overload
+    def __getitem__(self: _MST, i: int) -> _MST: ...
+    @overload
+    def __getitem__(self: _MST, s: slice) -> _MST: ...
     def __setitem__(self, index: Union[int, slice], sub: Any) -> None: ...
     def __delitem__(self, index: Union[int, slice]) -> None: ...
     def immutable(self) -> UserString: ...

stdlib/2/typing.pyi

@@ -163,6 +163,12 @@ class MutableSequence(Sequence[_T], Generic[_T]):
     def insert(self, index: int, object: _T) -> None: ...
     @overload
     @abstractmethod
+    def __getitem__(self, i: int) -> _T: ...
+    @overload
+    @abstractmethod
+    def __getitem__(self, s: slice) -> MutableSequence[_T]: ...
+    @overload
+    @abstractmethod
     def __setitem__(self, i: int, o: _T) -> None: ...
     @overload
     @abstractmethod

stdlib/2and3/xml/etree/ElementTree.pyi

@@ -74,7 +74,7 @@ class Element(MutableSequence[Element]):
     @overload
     def __getitem__(self, i: int) -> Element: ...
     @overload
-    def __getitem__(self, s: slice) -> Sequence[Element]: ...
+    def __getitem__(self, s: slice) -> MutableSequence[Element]: ...
     def __len__(self) -> int: ...
     @overload
     def __setitem__(self, i: int, o: Element) -> None: ...

stdlib/3/collections/__init__.pyi

@@ -86,7 +86,7 @@ class UserList(MutableSequence[_T]):
     @overload
     def __getitem__(self, i: int) -> _T: ...
     @overload
-    def __getitem__(self, i: slice) -> Sequence[_T]: ...
+    def __getitem__(self, i: slice) -> MutableSequence[_T]: ...
     @overload
     def __setitem__(self, i: int, o: _T) -> None: ...
     @overload
@@ -217,7 +217,7 @@ class deque(MutableSequence[_T], Generic[_T]):
     @overload
     def __getitem__(self, index: int) -> _T: ...
     @overload
-    def __getitem__(self, s: slice) -> Sequence[_T]:
+    def __getitem__(self, s: slice) -> MutableSequence[_T]:
         raise TypeError
     @overload
     def __setitem__(self, i: int, x: _T) -> None: ...

stdlib/3/typing.pyi

@@ -254,6 +254,12 @@ class MutableSequence(Sequence[_T], Generic[_T]):
     def insert(self, index: int, object: _T) -> None: ...
     @overload
     @abstractmethod
+    def __getitem__(self, i: int) -> _T: ...
+    @overload
+    @abstractmethod
+    def __getitem__(self, s: slice) -> MutableSequence[_T]: ...
+    @overload
+    @abstractmethod
     def __setitem__(self, i: int, o: _T) -> None: ...
     @overload
     @abstractmethod