bpo-41780: Fix __dir__ of types.GenericAlias (GH-22262)

Automerge-Triggered-By: @gvanrossum
(cherry picked from commit 2e87774df1a0eaf2a1fe8cc4d958df60f7125b6e)

Co-authored-by: Batuhan Taskaya <isidentical@gmail.com>

3 files changed, 46 insertions, 0 deletions

diff --git a/Lib/test/test_genericalias.py b/Lib/test/test_genericalias.py
index 4f3798e..ec1acd4 100644
--- a/Lib/test/test_genericalias.py
+++ b/Lib/test/test_genericalias.py
@@ -283,6 +283,11 @@ class BaseTest(unittest.TestCase):
         self.assertEqual(a.__args__, (list[T], tuple[T, ...]))
         self.assertEqual(a.__parameters__, (T,))
 
+    def test_dir(self):
+        dir_of_gen_alias = set(dir(list[int]))
+        self.assertTrue(dir_of_gen_alias.issuperset(dir(list)))
+        for generic_alias_property in ("__origin__", "__args__", "__parameters__"):
+            self.assertIn(generic_alias_property, dir_of_gen_alias)
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS.d/next/Core and Builtins/2020-09-15-23-29-49.bpo-41780.bOBUIH.rst b/Misc/NEWS.d/next/Core and Builtins/2020-09-15-23-29-49.bpo-41780.bOBUIH.rst
new file mode 100644
index 0000000..9a7594f
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2020-09-15-23-29-49.bpo-41780.bOBUIH.rst
@@ -0,0 +1,2 @@
+Fix :meth:`__dir__` of :class:`types.GenericAlias`. Patch by Batuhan
+Taskaya.
diff --git a/Objects/genericaliasobject.c b/Objects/genericaliasobject.c
index 4d511a2..4f95216 100644
--- a/Objects/genericaliasobject.c
+++ b/Objects/genericaliasobject.c
@@ -487,11 +487,50 @@ ga_reduce(PyObject *self, PyObject *Py_UNUSED(ignored))
                          alias->origin, alias->args);
 }
 
+static PyObject *
+ga_dir(PyObject *self, PyObject *Py_UNUSED(ignored))
+{
+    gaobject *alias = (gaobject *)self;
+    PyObject *dir = PyObject_Dir(alias->origin);
+    if (dir == NULL) {
+        return NULL;
+    }
+
+    PyObject *dir_entry = NULL;
+    for (const char * const *p = attr_exceptions; ; p++) {
+        if (*p == NULL) {
+            break;
+        }
+        else {
+            dir_entry = PyUnicode_FromString(*p);
+            if (dir_entry == NULL) {
+                goto error;
+            }
+            int contains = PySequence_Contains(dir, dir_entry);
+            if (contains < 0) {
+                goto error;
+            }
+            if (contains == 0 && PyList_Append(dir, dir_entry) < 0) {
+                goto error;
+            }
+
+            Py_CLEAR(dir_entry);
+        }
+    }
+    return dir;
+
+error:
+    Py_DECREF(dir);
+    Py_XDECREF(dir_entry);
+    return NULL;
+}
+
 static PyMethodDef ga_methods[] = {
     {"__mro_entries__", ga_mro_entries, METH_O},
     {"__instancecheck__", ga_instancecheck, METH_O},
     {"__subclasscheck__", ga_subclasscheck, METH_O},
     {"__reduce__", ga_reduce, METH_NOARGS},
+    {"__dir__", ga_dir, METH_NOARGS},
     {0}
 };