Export DataclassInstance protocol from _typeshed (#9676)

stdlib/_typeshed/__init__.pyi

@@ -8,9 +8,10 @@ import mmap
 import pickle
 import sys
 from collections.abc import Awaitable, Callable, Iterable, Set as AbstractSet
+from dataclasses import Field
 from os import PathLike
 from types import FrameType, TracebackType
-from typing import Any, AnyStr, Generic, Protocol, TypeVar, Union
+from typing import Any, AnyStr, ClassVar, Generic, Protocol, TypeVar, Union
 from typing_extensions import Final, Literal, LiteralString, TypeAlias, final
 
 _KT = TypeVar("_KT")
@@ -304,3 +305,10 @@ ProfileFunction: TypeAlias = Callable[[FrameType, str, Any], object]
 
 # Objects suitable to be passed to sys.settrace, threading.settrace, and similar
 TraceFunction: TypeAlias = Callable[[FrameType, str, Any], TraceFunction | None]
+
+# experimental
+# Might not work as expected for pyright, see
+#   https://github.com/python/typeshed/pull/9362
+#   https://github.com/microsoft/pyright/issues/4339
+class DataclassInstance(Protocol):
+    __dataclass_fields__: ClassVar[dict[str, Field[Any]]]