explicitly disallow unsupported base classes

tests/test_transformers.py

@@ -602,3 +602,35 @@ class MyPath(Path): ...
     """)
     with pytest.raises(NotImplementedError):
         transform_source(pyi_import_from)
+
+
+def test_subclass_object():
+    pyi_direct = _src("class OldStyle(object): ...")
+    with pytest.raises(NotImplementedError):
+        transform_source(pyi_direct)
+
+
+def test_subclass_builtins_object():
+    pyi_direct = _src("class OldStyle(__builtins__.object): ...")
+    with pytest.raises(NotImplementedError):
+        transform_source(pyi_direct)
+
+
+def test_subclass_builtins_object_import():
+    pyi_direct = _src("""
+    import builtins
+
+    class OldStyle(builtins.object): ...
+    """)
+    with pytest.raises(NotImplementedError):
+        transform_source(pyi_direct)
+
+
+def test_subclass_builtins_object_alias():
+    pyi_direct = _src("""
+    from builtins import object as Object
+
+    class OldStyle(Object): ...
+    """)
+    with pytest.raises(NotImplementedError):
+        transform_source(pyi_direct)

unpy/_stdlib.py

@@ -32,18 +32,20 @@
         "BaseExceptionGroup": (3, 11),
         "ExceptionGroup": (3, 11),
         "EncodingWarning": (3, 10),
+        "reveal_locals": (4, 0),
+        "reveal_type": (4, 0),
     },
     "enum": {
         # TODO(jorenham): Backport to `builtins.str & enum.Enum`
         "StrEnum": (3, 11),
     },
 }
 UNSUPPORTED_BASES: Final = {
-    "builtins": {"object": (4, 0)},
-    "inspect": {"BufferFlags", (3, 12)},
-    "pathlib": {"Path": (3, 12)},
-    "typing": {"Any": (3, 11)},
-    "typing_extensions": {"Any": (3, 11)},
+    "builtins.object": (4, 0),
+    "inspect.BufferFlags": (3, 12),
+    "pathlib.Path": (3, 12),
+    "typing.Any": (3, 11),
+    "typing_extensions.Any": (3, 11),
 }
 
 

unpy/transformers.py

@@ -13,7 +13,7 @@
 import libcst as cst
 
 import unpy._cst as uncst
-from unpy._stdlib import BACKPORTS
+from unpy._stdlib import BACKPORTS, UNSUPPORTED_BASES
 from unpy._types import PythonVersion
 from unpy.visitors import StubVisitor
 
@@ -124,6 +124,7 @@ def __init__(self, visitor: StubVisitor, /, target: PythonVersion) -> None:
         self._renames = {}
         self._type_alias_alignment = {}
 
+        self.__check_base_classes()
         self.__collect_imports_typevars()
         self.__collect_imports_backport()
         self.__collect_imports_generic()
@@ -187,6 +188,31 @@ def _discard_import(self, module: str, name: str, /) -> str | None:
 
         return None
 
+    def __check_base_classes(self, /) -> None:
+        # raise for unsupported base classes
+        target = self.target
+        visitor = self.visitor
+
+        illegal_fqn = {base for base, req in UNSUPPORTED_BASES.items() if target < req}
+        illegal_names = {
+            alias: base
+            for base in illegal_fqn
+            if (alias := visitor.imported_as(*base.rsplit(".", 1)))
+        }
+
+        for bases in self.visitor.class_bases.values():
+            for base in bases:
+                if base in illegal_names:
+                    raise NotImplementedError(f"{illegal_names[base]!r} as base class")
+                base = base.replace("__builtins__.", "builtins.")  # noqa: PLW2901
+                if (fqn := visitor.imports_by_alias.get(base, base)) in illegal_fqn:
+                    raise NotImplementedError(f"{fqn!r} as base class")
+                if base in visitor.imports_by_ref:
+                    module, name = visitor.imports_by_ref[base]
+                    fqn = f"{module}.{name}" if name else module
+                    if fqn in illegal_fqn:
+                        raise NotImplementedError(f"{fqn!r} as base class")
+
     def __collect_imports_typevars(self, /) -> None:
         # collect the missing imports for the typevar-likes
         target = self.target