More precise type hints for slice constructor

stdlib/builtins.pyi

@@ -9,7 +9,6 @@ from _typeshed import (
     ConvertibleToFloat,
     ConvertibleToInt,
     FileDescriptorOrPath,
-    MaybeNone,
     OpenBinaryMode,
     OpenBinaryModeReading,
     OpenBinaryModeUpdating,
@@ -95,9 +94,9 @@ _SupportsAnextT = TypeVar("_SupportsAnextT", bound=SupportsAnext[Any], covariant
 _AwaitableT = TypeVar("_AwaitableT", bound=Awaitable[Any])
 _AwaitableT_co = TypeVar("_AwaitableT_co", bound=Awaitable[Any], covariant=True)
 _P = ParamSpec("_P")
-_StartT = TypeVar("_StartT", covariant=True, default=Any)
-_StopT = TypeVar("_StopT", covariant=True, default=Any)
-_StepT = TypeVar("_StepT", covariant=True, default=Any)
+_StartT_co = TypeVar("_StartT_co", covariant=True, default=Any)
+_StopT_co = TypeVar("_StopT_co", covariant=True, default=_StartT_co)  #  slice[A] -> slice[A, A, Any]
+_StepT_co = TypeVar("_StepT_co", covariant=True, default=Any)  #  slice[A,B] -> slice[A, B, Any]
 
 class object:
     __doc__: str | None
@@ -940,24 +939,34 @@ class bool(int):
     def __invert__(self) -> int: ...
 
 @final
-class slice(Generic[_StartT, _StopT, _StepT]):
+class slice(Generic[_StartT_co, _StopT_co, _StepT_co]):
     @property
-    def start(self) -> _StartT: ...
+    def start(self) -> _StartT_co: ...
     @property
-    def step(self) -> _StepT: ...
+    def step(self) -> _StepT_co: ...
     @property
-    def stop(self) -> _StopT: ...
-    @overload
-    def __new__(cls, stop: int | None, /) -> slice[int | MaybeNone, int | MaybeNone, int | MaybeNone]: ...
-    @overload
-    def __new__(
-        cls, start: int | None, stop: int | None, step: int | None = None, /
-    ) -> slice[int | MaybeNone, int | MaybeNone, int | MaybeNone]: ...
-    @overload
-    def __new__(cls, stop: _T2, /) -> slice[Any, _T2, Any]: ...
-    @overload
-    def __new__(cls, start: _T1, stop: _T2, /) -> slice[_T1, _T2, Any]: ...
-    @overload
+    def stop(self) -> _StopT_co: ...
+    # generic slice --------------------------------------------------------------------
+    @overload  # 2x None
+    def __new__(cls, start: None, stop: None = None, step: None = None, /) -> slice[Any, Any, Any]: ...
+    # unary overloads ------------------------------------------------------------------
+    @overload  # 0x None
+    def __new__(cls, stop: _T2, /) -> slice[_T2 | None, _T2 | None, Any]: ...
+    # binary overloads -----------------------------------------------------------------
+    @overload  # 1x None
+    def __new__(cls, start: _T1, stop: None, step: None = None, /) -> slice[_T1 | None, _T1 | None, Any]: ...
+    @overload  # 1x None
+    def __new__(cls, start: None, stop: _T2, step: None = None, /) -> slice[_T2 | None, _T2 | None, Any]: ...
+    @overload  # 0x None
+    def __new__(cls, start: _T1, stop: _T2, step: None = None, /) -> slice[_T1, _T2, Any]: ...
+    # ternary overloads ----------------------------------------------------------------
+    @overload  # 2x None
+    def __new__(cls, start: None, stop: None, step: _T3, /) -> slice[Any, Any, _T3]: ...
+    @overload  # 1x None
+    def __new__(cls, start: _T1, stop: None, step: _T3, /) -> slice[_T1 | None, _T1 | None, _T3]: ...
+    @overload  # 1x None
+    def __new__(cls, start: None, stop: _T2, step: _T3, /) -> slice[_T2 | None, _T2 | None, _T3]: ...
+    @overload  # 0x None
     def __new__(cls, start: _T1, stop: _T2, step: _T3, /) -> slice[_T1, _T2, _T3]: ...
     def __eq__(self, value: object, /) -> bool: ...
     if sys.version_info >= (3, 12):