gh-101162: Forbid using issubclass with GenericAlias as the 1st arg

Lib/test/test_typing.py

@@ -4091,6 +4091,20 @@ class C(Generic[T]): pass
         with self.assertRaises(TypeError):
             C[()]
 
+    def test_generic_subclass_checks(self):
+        for typ in [list[int], List[int], tuple[int, str], Tuple[int, str],
+                    dict[str, int], Dict[str, int], set[str], typing.Set[str]]:
+            with self.subTest(typ=typ):
+                self.assertRaises(TypeError, issubclass, typ, object)
+                self.assertRaises(TypeError, issubclass, typ, type)
+                self.assertRaises(TypeError, issubclass, typ, typ)
+
+                # isinstance is fine:
+                self.assertTrue(isinstance(typ, object))
+                self.assertTrue(isinstance(typ, object))
+                # but, not when the right arg is also a generic:
+                self.assertRaises(TypeError, isinstance, typ, typ)
+
     def test_init(self):
         T = TypeVar('T')
         S = TypeVar('S')

Misc/NEWS.d/next/Library/2023-04-08-12-43-52.gh-issue-101162.yOCd_J.rst

@@ -0,0 +1,2 @@
+Forbid using :func:`builtins.issubclass` with :class:`types.GenericAlias` as
+the first argument.

Objects/abstract.c

@@ -2812,7 +2812,7 @@ object_issubclass(PyThreadState *tstate, PyObject *derived, PyObject *cls)
         return -1;
     }
 
-    /* Probably never reached anymore. */
+    /* Can be reached when infinite recursion happens. */
     return recursive_issubclass(derived, cls);
 }
 

Objects/genericaliasobject.c

@@ -626,6 +626,7 @@ ga_vectorcall(PyObject *self, PyObject *const *args,
 
 static const char* const attr_exceptions[] = {
     "__class__",
+    "__bases__",
     "__origin__",
     "__args__",
     "__unpacked__",