Initial exploration and issue reproduction

Co-authored-by: Pierre-Sassoulas <[email protected]>

Author: Copilot

test_issue.py

@@ -0,0 +1,33 @@
+"""Playground file. Do not commit"""
+
+from abc import ABC
+from collections.abc import Callable
+from dataclasses import dataclass, field
+from typing import ParamSpec
+
+_P = ParamSpec("_P")
+
+
+@dataclass
+class Foo[T](ABC):
+
+    _foo: T | None = field(init=False)
+    _bar: dict[str, str] = field(init=False)
+
+    def __init_subclass__(cls) -> None:
+        def _wrap(func: Callable[_P, None]) -> Callable[_P, None]:
+            def _w(*args: _P.args, **kwds: _P.kwargs) -> None:
+                self = args[0]
+                func(*args, **kwds)
+                if not hasattr(self, "_foo"):
+                    object.__setattr__(self, "_foo", None)
+                if not hasattr(self, "_bar"):
+                    object.__setattr__(self, "_bar", {})
+
+            return _w
+
+        cls.__init__ = _wrap(cls.__init__)  # type: ignore[method-assign]
+
+
+@dataclass
+class Bar(Foo): ...