From 87b91c0bf6558ee1992cca0b56dbd0e076181236 Mon Sep 17 00:00:00 2001
From: sobolevn <mail@sobolevn.me>
Date: Wed, 25 Sep 2024 12:15:46 +0300
Subject: [PATCH 1/2] gh-124498: Fix `TypeAliasType` not to be generic, when
 `type_params=()`

---
 Lib/test/test_type_aliases.py                       | 13 +++++++++++++
 .../2024-09-25-12-14-58.gh-issue-124498.Ozxs55.rst  |  2 ++
 Objects/typevarobject.c                             |  3 ++-
 3 files changed, 17 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2024-09-25-12-14-58.gh-issue-124498.Ozxs55.rst

diff --git a/Lib/test/test_type_aliases.py b/Lib/test/test_type_aliases.py
index 49d6aa810304fb..254f3803f8f41e 100644
--- a/Lib/test/test_type_aliases.py
+++ b/Lib/test/test_type_aliases.py
@@ -211,6 +211,19 @@ def test_generic(self):
         self.assertEqual(TA.__value__, list[T])
         self.assertEqual(TA.__type_params__, (T,))
         self.assertEqual(TA.__module__, __name__)
+        self.assertIs(TA[int].__class__, types.GenericAlias)
+
+    def test_not_generic(self):
+        TA = TypeAliasType("TA", list[int], type_params=())
+        self.assertEqual(TA.__name__, "TA")
+        self.assertEqual(TA.__value__, list[int])
+        self.assertEqual(TA.__type_params__, ())
+        self.assertEqual(TA.__module__, __name__)
+        with self.assertRaisesRegex(
+            TypeError,
+            "Only generic type aliases are subscriptable",
+        ):
+            TA[int]
 
     def test_keywords(self):
         TA = TypeAliasType(name="TA", value=int)
diff --git a/Misc/NEWS.d/next/Library/2024-09-25-12-14-58.gh-issue-124498.Ozxs55.rst b/Misc/NEWS.d/next/Library/2024-09-25-12-14-58.gh-issue-124498.Ozxs55.rst
new file mode 100644
index 00000000000000..4dbf4eb709733d
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-09-25-12-14-58.gh-issue-124498.Ozxs55.rst
@@ -0,0 +1,2 @@
+Fix :class:`typing.TypeAliasType` not to be generic, when ``type_params`` is
+an empty tuple.
diff --git a/Objects/typevarobject.c b/Objects/typevarobject.c
index d3656155fae330..6c2361f076c24c 100644
--- a/Objects/typevarobject.c
+++ b/Objects/typevarobject.c
@@ -1957,7 +1957,8 @@ typealias_reduce_impl(typealiasobject *self)
 static PyObject *
 typealias_subscript(PyObject *self, PyObject *args)
 {
-    if (((typealiasobject *)self)->type_params == NULL) {
+    typealiasobject *ta = ((typealiasobject *)self);
+    if (ta->type_params == NULL || PyTuple_GET_SIZE(ta->type_params) == 0) {
         PyErr_SetString(PyExc_TypeError,
                         "Only generic type aliases are subscriptable");
         return NULL;

From 210592519186e662eaee31117e09866d138bcb8e Mon Sep 17 00:00:00 2001
From: sobolevn <mail@sobolevn.me>
Date: Thu, 26 Sep 2024 15:27:57 +0300
Subject: [PATCH 2/2] Address review

---
 Lib/test/test_type_aliases.py |  2 +-
 Objects/typevarobject.c       | 14 +++++++++++---
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/Lib/test/test_type_aliases.py b/Lib/test/test_type_aliases.py
index 254f3803f8f41e..ebb65d8c6cf81b 100644
--- a/Lib/test/test_type_aliases.py
+++ b/Lib/test/test_type_aliases.py
@@ -211,7 +211,7 @@ def test_generic(self):
         self.assertEqual(TA.__value__, list[T])
         self.assertEqual(TA.__type_params__, (T,))
         self.assertEqual(TA.__module__, __name__)
-        self.assertIs(TA[int].__class__, types.GenericAlias)
+        self.assertIs(type(TA[int]), types.GenericAlias)
 
     def test_not_generic(self):
         TA = TypeAliasType("TA", list[int], type_params=())
diff --git a/Objects/typevarobject.c b/Objects/typevarobject.c
index 6c2361f076c24c..b01c6dc705930c 100644
--- a/Objects/typevarobject.c
+++ b/Objects/typevarobject.c
@@ -1914,7 +1914,16 @@ typealias_alloc(PyObject *name, PyObject *type_params, PyObject *compute_value,
         return NULL;
     }
     ta->name = Py_NewRef(name);
-    ta->type_params = Py_IsNone(type_params) ? NULL : Py_XNewRef(type_params);
+    if (
+        type_params == NULL
+        || Py_IsNone(type_params)
+        || (PyTuple_Check(type_params) && PyTuple_GET_SIZE(type_params) == 0)
+    ) {
+        ta->type_params = NULL;
+    }
+    else {
+        ta->type_params = Py_NewRef(type_params);
+    }
     ta->compute_value = Py_XNewRef(compute_value);
     ta->value = Py_XNewRef(value);
     ta->module = Py_XNewRef(module);
@@ -1957,8 +1966,7 @@ typealias_reduce_impl(typealiasobject *self)
 static PyObject *
 typealias_subscript(PyObject *self, PyObject *args)
 {
-    typealiasobject *ta = ((typealiasobject *)self);
-    if (ta->type_params == NULL || PyTuple_GET_SIZE(ta->type_params) == 0) {
+    if (((typealiasobject *)self)->type_params == NULL) {
         PyErr_SetString(PyExc_TypeError,
                         "Only generic type aliases are subscriptable");
         return NULL;