[3.13] gh-124498: Fix TypeAliasType not to be generic, when type_params=() (GH-124499) (#124603)

gh-124498: Fix `TypeAliasType` not to be generic, when `type_params=()` (GH-124499)
(cherry picked from commit abe5f79)

Co-authored-by: sobolevn <mail@sobolevn.me>

Author: miss-islington

Diff for: Lib/test/test_type_aliases.py

@@ -212,6 +212,19 @@ def test_generic(self):
         self.assertEqual(TA.__value__, list[T])
         self.assertEqual(TA.__type_params__, (T,))
         self.assertEqual(TA.__module__, __name__)
+        self.assertIs(type(TA[int]), 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 for: 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 for: Objects/typevarobject.c

@@ -1640,7 +1640,16 @@ 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);