Fix copy.replace widening a bound TypeVar to its bound

The covariant protocol from #14786 loses a bound TypeVar whose __replace__
returns Self (dataclasses, namedtuples). Add a Self overload that keeps the
argument's type, falling back to the covariant one so Box[int] -> Box[str]
still works.

Author: apoorva-01

stdlib/@tests/test_cases/check_copy.py

@@ -2,6 +2,7 @@
 
 import copy
 import sys
+from dataclasses import dataclass
 from typing import Generic, TypeVar
 from typing_extensions import Self, assert_type
 
@@ -37,3 +38,28 @@ def __replace__(self, value: str) -> Box[str]:
     box1: Box[int] = Box(42)
     box2 = copy.replace(box1, val="spam")
     assert_type(box2, Box[str])
+
+
+# Regression test for #15973: replace() must preserve a bound TypeVar whose
+# __replace__ returns Self (the dataclass/namedtuple case), rather than widening
+# to the bound.
+@dataclass(frozen=True)
+class BaseConfig:
+    pg_ssl_key: str | None = None
+
+
+@dataclass(frozen=True)
+class SubConfig(BaseConfig):
+    pg_host: str = "localhost"
+
+
+_ConfigT = TypeVar("_ConfigT", bound=BaseConfig)
+
+if sys.version_info >= (3, 13):
+
+    def replace_config(config: _ConfigT) -> _ConfigT:
+        result = copy.replace(config, pg_ssl_key="replaced")
+        assert_type(result, _ConfigT)
+        return result
+
+    assert_type(copy.replace(SubConfig(), pg_host="example.com"), SubConfig)

stdlib/copy.pyi

@@ -1,16 +1,23 @@
 import sys
-from typing import Any, Protocol, TypeVar, type_check_only
+from typing import Any, Protocol, TypeVar, overload, type_check_only
+from typing_extensions import Self
 
 __all__ = ["Error", "copy", "deepcopy"]
 
 _T = TypeVar("_T")
 _RT_co = TypeVar("_RT_co", covariant=True)
 
 @type_check_only
-class _SupportsReplace(Protocol[_RT_co]):
+class _SupportsReplaceSelf(Protocol):
     # In reality doesn't support args, but there's no great way to express this.
+    def __replace__(self, /, *_: Any, **changes: Any) -> Self: ...
+
+@type_check_only
+class _SupportsReplace(Protocol[_RT_co]):
     def __replace__(self, /, *_: Any, **changes: Any) -> _RT_co: ...
 
+_SR = TypeVar("_SR", bound=_SupportsReplaceSelf)
+
 # None in CPython but non-None in Jython
 PyStringMap: Any
 
@@ -21,6 +28,11 @@ def copy(x: _T) -> _T: ...
 if sys.version_info >= (3, 13):
     __all__ += ["replace"]
     # The types accepted by `**changes` match those of `obj.__replace__`.
+    # When `__replace__` returns `Self`, keep the argument's own type (so a bound
+    # TypeVar is preserved); otherwise return whatever `__replace__` declares.
+    @overload
+    def replace(obj: _SR, /, **changes: Any) -> _SR: ...
+    @overload
     def replace(obj: _SupportsReplace[_RT_co], /, **changes: Any) -> _RT_co: ...
 
 class Error(Exception): ...