Make BufferedReader generic over a protocol

stdlib/@tests/test_cases/check_io.py

@@ -1,7 +1,10 @@
+from _io import BufferedReader
 from gzip import GzipFile
-from io import FileIO, TextIOWrapper
+from io import FileIO, RawIOBase, TextIOWrapper
 from typing_extensions import assert_type
 
+BufferedReader(RawIOBase())
+
 assert_type(TextIOWrapper(FileIO("")).buffer, FileIO)
 assert_type(TextIOWrapper(FileIO(13)).detach(), FileIO)
 assert_type(TextIOWrapper(GzipFile("")).buffer, GzipFile)

stdlib/_io.pyi

@@ -88,9 +88,36 @@ class BytesIO(BufferedIOBase, _BufferedIOBase, BinaryIO):  # type: ignore[misc]
     def readlines(self, size: int | None = None, /) -> list[bytes]: ...
     def seek(self, pos: int, whence: int = 0, /) -> int: ...
 
-class BufferedReader(BufferedIOBase, _BufferedIOBase, BinaryIO):  # type: ignore[misc]  # incompatible definitions of methods in the base classes
-    raw: RawIOBase
-    def __init__(self, raw: RawIOBase, buffer_size: int = 8192) -> None: ...
+class _BufferedReaderStream(Protocol):
+    def read(self, n: int = ..., /) -> bytes: ...
+    # Optional: def readall(self) -> bytes: ...
+    def readinto(self, b: memoryview, /) -> int | None: ...
+    def seek(self, pos: int, whence: int, /) -> int: ...
+    def tell(self) -> int: ...
+    def truncate(self, size: int, /) -> int: ...
+    def flush(self) -> object: ...
+    def close(self) -> object: ...
+    @property
+    def closed(self) -> bool: ...
+    def readable(self) -> bool: ...
+    def seekable(self) -> bool: ...
+
+    # The following methods just pass through to the underlying stream. Since
+    # not all streams support them, they are marked as optional here, and will
+    # raise an AttributeError if called on a stream that does not support them.
+
+    # @property
+    # def name(self) -> Any: ...  # Type is inconsistent between the various I/O types.
+    # @property
+    # def mode(self) -> str: ...
+    # def fileno(self) -> int: ...
+    # def isatty(self) -> bool: ...
+
+_BufferedReaderStreamT = TypeVar("_BufferedReaderStreamT", bound=_BufferedReaderStream, default=_BufferedReaderStream)
+
+class BufferedReader(BufferedIOBase, _BufferedIOBase, BinaryIO, Generic[_BufferedReaderStreamT]):  # type: ignore[misc]  # incompatible definitions of methods in the base classes
+    raw: _BufferedReaderStreamT
+    def __init__(self, raw: _BufferedReaderStreamT, buffer_size: int = 8192) -> None: ...
     def peek(self, size: int = 0, /) -> bytes: ...
     def seek(self, target: int, whence: int = 0, /) -> int: ...
     def truncate(self, pos: int | None = None, /) -> int: ...
@@ -111,8 +138,8 @@ class BufferedRandom(BufferedIOBase, _BufferedIOBase, BinaryIO):  # type: ignore
     def peek(self, size: int = 0, /) -> bytes: ...
     def truncate(self, pos: int | None = None, /) -> int: ...
 
-class BufferedRWPair(BufferedIOBase, _BufferedIOBase):
-    def __init__(self, reader: RawIOBase, writer: RawIOBase, buffer_size: int = 8192, /) -> None: ...
+class BufferedRWPair(BufferedIOBase, _BufferedIOBase, Generic[_BufferedReaderStreamT]):
+    def __init__(self, reader: _BufferedReaderStreamT, writer: RawIOBase, buffer_size: int = 8192, /) -> None: ...
     def peek(self, size: int = 0, /) -> bytes: ...
 
 class _TextIOBase(_IOBase):
@@ -131,8 +158,7 @@ class _TextIOBase(_IOBase):
 @type_check_only
 class _WrappedBuffer(Protocol):
     # "name" is wrapped by TextIOWrapper. Its type is inconsistent between
-    # the various I/O types, see the comments on TextIOWrapper.name and
-    # TextIO.name.
+    # the various I/O types.
     @property
     def name(self) -> Any: ...
     @property