typing: Use PEP 695 syntax in typing.py (python#104553)

Co-authored-by: Alex Waygood <[email protected]>

Author: JelleZijlstra

Lib/typing.py

@@ -2471,8 +2471,9 @@ class Other(Leaf):  # Error reported by type checker
     return f
 
 
-# Some unconstrained type variables.  These are used by the container types.
-# (These are not for export.)
+# Some unconstrained type variables.  These were initially used by the container types.
+# They were never meant for export and are now unused, but we keep them around to
+# avoid breaking compatibility with users who import them.
 T = TypeVar('T')  # Any type.
 KT = TypeVar('KT')  # Key type.
 VT = TypeVar('VT')  # Value type.
@@ -2577,8 +2578,6 @@ def new_user(user_class: Type[U]) -> U:
     At this point the type checker knows that joe has type BasicUser.
     """
 
-# Internal type variable for callables. Not for export.
-F = TypeVar("F", bound=Callable[..., Any])
 
 @runtime_checkable
 class SupportsInt(Protocol):
@@ -2631,22 +2630,22 @@ def __index__(self) -> int:
 
 
 @runtime_checkable
-class SupportsAbs(Protocol[T_co]):
+class SupportsAbs[T](Protocol):
     """An ABC with one abstract method __abs__ that is covariant in its return type."""
     __slots__ = ()
 
     @abstractmethod
-    def __abs__(self) -> T_co:
+    def __abs__(self) -> T:
         pass
 
 
 @runtime_checkable
-class SupportsRound(Protocol[T_co]):
+class SupportsRound[T](Protocol):
     """An ABC with one abstract method __round__ that is covariant in its return type."""
     __slots__ = ()
 
     @abstractmethod
-    def __round__(self, ndigits: int = 0) -> T_co:
+    def __round__(self, ndigits: int = 0) -> T:
         pass
 
 
@@ -3183,7 +3182,7 @@ class re(metaclass=_DeprecatedType):
 sys.modules[re.__name__] = re
 
 
-def reveal_type(obj: T, /) -> T:
+def reveal_type[T](obj: T, /) -> T:
     """Reveal the inferred type of a variable.
 
     When a static type checker encounters a call to ``reveal_type()``,
@@ -3203,6 +3202,11 @@ def reveal_type(obj: T, /) -> T:
     return obj
 
 
+class _IdentityCallable(Protocol):
+    def __call__[T](self, arg: T, /) -> T:
+        ...
+
+
 def dataclass_transform(
     *,
     eq_default: bool = True,
@@ -3211,7 +3215,7 @@ def dataclass_transform(
     frozen_default: bool = False,
     field_specifiers: tuple[type[Any] | Callable[..., Any], ...] = (),
     **kwargs: Any,
-) -> Callable[[T], T]:
+) -> _IdentityCallable:
     """Decorator that marks a function, class, or metaclass as providing
     dataclass-like behavior.
 
@@ -3288,8 +3292,10 @@ def decorator(cls_or_fn):
     return decorator
 
 
+type _Func = Callable[..., Any]
+
 
-def override(method: F, /) -> F:
+def override[F: _Func](method: F, /) -> F:
     """Indicate that a method is intended to override a method in a base class.
 
     Usage: