[3.9] bpo-42332: Add weakref slot to types.GenericAlias (GH-23250) (GH-23309)

(cherry picked from commit 384b7a4bd988986bca227c7e85c32d766da74708)

3 files changed, 59 insertions, 38 deletions

diff --git a/Lib/test/test_genericalias.py b/Lib/test/test_genericalias.py
index 240aad0..912fb33 100644
--- a/Lib/test/test_genericalias.py
+++ b/Lib/test/test_genericalias.py
@@ -13,7 +13,10 @@ from contextvars import ContextVar, Token
 from dataclasses import Field
 from functools import partial, partialmethod, cached_property
 from mailbox import Mailbox, _PartialFile
-from ctypes import Array, LibraryLoader
+try:
+    import ctypes
+except ImportError:
+    ctypes = None
 from difflib import SequenceMatcher
 from filecmp import dircmp
 from fileinput import FileInput
@@ -44,45 +47,46 @@ V = TypeVar('V')
 
 class BaseTest(unittest.TestCase):
     """Test basics."""
+    generic_types = [type, tuple, list, dict, set, frozenset, enumerate,
+                     defaultdict, deque,
+                     SequenceMatcher,
+                     dircmp,
+                     FileInput,
+                     OrderedDict, Counter, UserDict, UserList,
+                     Pattern, Match,
+                     partial, partialmethod, cached_property,
+                     AbstractContextManager, AbstractAsyncContextManager,
+                     Awaitable, Coroutine,
+                     AsyncIterable, AsyncIterator,
+                     AsyncGenerator, Generator,
+                     Iterable, Iterator,
+                     Reversible,
+                     Container, Collection,
+                     Callable,
+                     Mailbox, _PartialFile,
+                     ContextVar, Token,
+                     Field,
+                     Set, MutableSet,
+                     Mapping, MutableMapping, MappingView,
+                     KeysView, ItemsView, ValuesView,
+                     Sequence, MutableSequence,
+                     MappingProxyType, AsyncGeneratorType,
+                     DirEntry,
+                     chain,
+                     TemporaryDirectory, SpooledTemporaryFile,
+                     Queue, SimpleQueue,
+                     _AssertRaisesContext,
+                     SplitResult, ParseResult,
+                     ValueProxy, ApplyResult,
+                     WeakSet, ReferenceType, ref,
+                     ShareableList, MPSimpleQueue,
+                     Future, _WorkItem,
+                     Morsel]
+    if ctypes is not None:
+        generic_types.extend((ctypes.Array, ctypes.LibraryLoader))
 
     def test_subscriptable(self):
-        for t in (type, tuple, list, dict, set, frozenset, enumerate,
-                  defaultdict, deque,
-                  SequenceMatcher,
-                  dircmp,
-                  FileInput,
-                  OrderedDict, Counter, UserDict, UserList,
-                  Pattern, Match,
-                  partial, partialmethod, cached_property,
-                  AbstractContextManager, AbstractAsyncContextManager,
-                  Awaitable, Coroutine,
-                  AsyncIterable, AsyncIterator,
-                  AsyncGenerator, Generator,
-                  Iterable, Iterator,
-                  Reversible,
-                  Container, Collection,
-                  Callable,
-                  Mailbox, _PartialFile,
-                  ContextVar, Token,
-                  Field,
-                  Set, MutableSet,
-                  Mapping, MutableMapping, MappingView,
-                  KeysView, ItemsView, ValuesView,
-                  Sequence, MutableSequence,
-                  MappingProxyType, AsyncGeneratorType,
-                  DirEntry,
-                  chain,
-                  TemporaryDirectory, SpooledTemporaryFile,
-                  Queue, SimpleQueue,
-                  _AssertRaisesContext,
-                  Array, LibraryLoader,
-                  SplitResult, ParseResult,
-                  ValueProxy, ApplyResult,
-                  WeakSet, ReferenceType, ref,
-                  ShareableList, MPSimpleQueue,
-                  Future, _WorkItem,
-                  Morsel,
-                  ):
+        for t in self.generic_types:
             if t is None:
                 continue
             tname = t.__name__
@@ -289,5 +293,15 @@ class BaseTest(unittest.TestCase):
         for generic_alias_property in ("__origin__", "__args__", "__parameters__"):
             self.assertIn(generic_alias_property, dir_of_gen_alias)
 
+    def test_weakref(self):
+        for t in self.generic_types:
+            if t is None:
+                continue
+            tname = t.__name__
+            with self.subTest(f"Testing {tname}"):
+                alias = t[int]
+                self.assertEqual(ref(alias)(), alias)
+
+
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS.d/next/Core and Builtins/2020-11-12-23-16-14.bpo-42332.fEQIdk.rst b/Misc/NEWS.d/next/Core and Builtins/2020-11-12-23-16-14.bpo-42332.fEQIdk.rst
new file mode 100644
index 0000000..8a2cb87
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2020-11-12-23-16-14.bpo-42332.fEQIdk.rst
@@ -0,0 +1 @@
+:class:`types.GenericAlias` objects can now be the targets of weakrefs.
diff --git a/Objects/genericaliasobject.c b/Objects/genericaliasobject.c
index 3e850b5..4b6c8c6 100644
--- a/Objects/genericaliasobject.c
+++ b/Objects/genericaliasobject.c
@@ -9,6 +9,7 @@ typedef struct {
     PyObject *origin;
     PyObject *args;
     PyObject *parameters;
+    PyObject* weakreflist;
 } gaobject;
 
 static void
@@ -17,6 +18,9 @@ ga_dealloc(PyObject *self)
     gaobject *alias = (gaobject *)self;
 
     _PyObject_GC_UNTRACK(self);
+    if (alias->weakreflist != NULL) {
+        PyObject_ClearWeakRefs((PyObject *)alias);
+    }
     Py_XDECREF(alias->origin);
     Py_XDECREF(alias->args);
     Py_XDECREF(alias->parameters);
@@ -593,6 +597,7 @@ PyTypeObject Py_GenericAliasType = {
     .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
     .tp_traverse = ga_traverse,
     .tp_richcompare = ga_richcompare,
+    .tp_weaklistoffset = offsetof(gaobject, weakreflist),
     .tp_methods = ga_methods,
     .tp_members = ga_members,
     .tp_alloc = PyType_GenericAlloc,
@@ -624,6 +629,7 @@ Py_GenericAlias(PyObject *origin, PyObject *args)
     alias->origin = origin;
     alias->args = args;
     alias->parameters = NULL;
+    alias->weakreflist = NULL;
     _PyObject_GC_TRACK(alias);
     return (PyObject *)alias;
 }