[functools] Make partial covariant in its return type

stdlib/@tests/test_cases/check_functools.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from functools import cache, cached_property, lru_cache, singledispatchmethod, update_wrapper, wraps
+from functools import cache, cached_property, lru_cache, partial, singledispatchmethod, update_wrapper, wraps
 from typing import Callable, ParamSpec, TypeVar
 from typing_extensions import assert_type
 
@@ -194,6 +194,20 @@ def method(self) -> Child:
         return Child()
 
 
+def _returns_child() -> Child:
+    return Child()
+
+
+def _accept_parent_partial(p: partial[Parent]) -> None:
+    assert_type(p(), Parent)
+
+
+def check_partial_covariance() -> None:
+    p_child = partial(_returns_child)
+    assert_type(p_child, partial[Child])
+    _accept_parent_partial(p_child)
+
+
 class Formatter:
     @singledispatchmethod
     def format(self, value: object) -> str:

stdlib/functools.pyi

@@ -183,20 +183,20 @@ def total_ordering(cls: type[_T]) -> type[_T]: ...
 def cmp_to_key(mycmp: Callable[[_T, _T], int]) -> Callable[[_T], SupportsAllComparisons]: ...
 
 @disjoint_base
-class partial(Generic[_T]):
+class partial(Generic[_T_co]):
     @property
-    def func(self) -> Callable[..., _T]: ...
+    def func(self) -> Callable[..., _T_co]: ...
     @property
     def args(self) -> tuple[Any, ...]: ...
     @property
     def keywords(self) -> dict[str, Any]: ...
-    def __new__(cls, func: Callable[..., _T], /, *args: Any, **kwargs: Any) -> Self: ...
-    def __call__(self, /, *args: Any, **kwargs: Any) -> _T: ...
+    def __new__(cls, func: Callable[..., _T_co], /, *args: Any, **kwargs: Any) -> Self: ...
+    def __call__(self, /, *args: Any, **kwargs: Any) -> _T_co: ...
     if sys.version_info >= (3, 14):
         @overload
         def __get__(self, instance: None, owner: type[Any] | None = None, /) -> Self: ...
         @overload
-        def __get__(self, instance: object, owner: type[Any] | None = None, /) -> Callable[..., _T]: ...
+        def __get__(self, instance: object, owner: type[Any] | None = None, /) -> Callable[..., _T_co]: ...
 
     def __class_getitem__(cls, item: Any, /) -> GenericAlias: ...