Refine the copy._SupportsReplace.__replace__ signature (#14786)

Author: jorenham

stdlib/@tests/test_cases/check_copy.py

@@ -2,6 +2,7 @@
 
 import copy
 import sys
+from typing import Generic, TypeVar
 from typing_extensions import Self, assert_type
 
 
@@ -19,3 +20,20 @@ def __replace__(self, val: int) -> Self:
     obj = ReplaceableClass(42)
     cpy = copy.replace(obj, val=23)
     assert_type(cpy, ReplaceableClass)
+
+
+_T_co = TypeVar("_T_co", covariant=True)
+
+
+class Box(Generic[_T_co]):
+    def __init__(self, value: _T_co, /) -> None:
+        self.value = value
+
+    def __replace__(self, value: str) -> Box[str]:
+        return Box(value)
+
+
+if sys.version_info >= (3, 13):
+    box1: Box[int] = Box(42)
+    box2 = copy.replace(box1, val="spam")
+    assert_type(box2, Box[str])

stdlib/copy.pyi

@@ -1,16 +1,15 @@
 import sys
 from typing import Any, Protocol, TypeVar, type_check_only
-from typing_extensions import Self
 
 __all__ = ["Error", "copy", "deepcopy"]
 
 _T = TypeVar("_T")
-_SR = TypeVar("_SR", bound=_SupportsReplace)
+_RT_co = TypeVar("_RT_co", covariant=True)
 
 @type_check_only
-class _SupportsReplace(Protocol):
-    # In reality doesn't support args, but there's no other great way to express this.
-    def __replace__(self, *args: Any, **kwargs: Any) -> Self: ...
+class _SupportsReplace(Protocol[_RT_co]):
+    # In reality doesn't support args, but there's no great way to express this.
+    def __replace__(self, /, *_: Any, **changes: Any) -> _RT_co: ...
 
 # None in CPython but non-None in Jython
 PyStringMap: Any
@@ -21,7 +20,8 @@ def copy(x: _T) -> _T: ...
 
 if sys.version_info >= (3, 13):
     __all__ += ["replace"]
-    def replace(obj: _SR, /, **changes: Any) -> _SR: ...
+    # The types accepted by `**changes` match those of `obj.__replace__`.
+    def replace(obj: _SupportsReplace[_RT_co], /, **changes: Any) -> _RT_co: ...
 
 class Error(Exception): ...