gh-158981: Fix subscripting generic aliases that omit a parameter with a default

Make typing._unpack_args() return a tuple, like its C counterpart in
Objects/genericaliasobject.c, so that TypeVar and TypeVarTuple defaults
are applied correctly.

Author: sricursion

Lib/test/test_typing.py

@@ -737,6 +737,28 @@ def test_allow_default_after_non_default_in_alias(self):
         a4 = Callable[*Ts, T]
         self.assertEqual(a4.__args__, (*Ts, T))
 
+    def test_typevar_default_in_alias_specialization(self):
+        T = TypeVar('T')
+        D = TypeVar('D', default=int)
+        class A(Generic[T, D]): ...
+
+        self.assertEqual(Dict[T, D][str], Dict[str, int])
+        self.assertEqual(Tuple[T, D][str], Tuple[str, int])
+        self.assertEqual(Callable[[T], D][str], Callable[[str], int])
+        self.assertEqual(A[T, D][str].__args__, (str, int))
+        self.assertEqual(dict[T, D][str], dict[str, int])
+        self.assertEqual(Dict[T, D][str, bytes], Dict[str, bytes])
+
+    def test_typevartuple_default_in_alias_specialization(self):
+        T = TypeVar('T')
+        Ts = TypeVarTuple('Ts', default=Unpack[Tuple[int, str]])
+
+        self.assertEqual(Tuple[bool, *Ts][()].__args__, (bool, int, str))
+        self.assertEqual(tuple[bool, *Ts][()].__args__, (bool, int, str))
+        self.assertEqual(tuple[*Ts][()].__args__, (int, str))
+        self.assertEqual(tuple[T, *Ts][bool].__args__, (bool, int, str))
+        self.assertEqual(tuple[T, *Ts][bool, bytes].__args__, (bool, bytes))
+
     def test_paramspec_specialization(self):
         T = TypeVar("T")
         P = ParamSpec('P', default=[str, int])

Lib/typing.py

@@ -363,7 +363,7 @@ def _unpack_args(*args):
             newargs.extend(subargs)
         else:
             newargs.append(arg)
-    return newargs
+    return tuple(newargs)
 
 def _deduplicate(params, *, unhashable_fallback=False):
     # Weed out strict duplicates, preserving the first of each occurrence.

Misc/NEWS.d/next/Library/2026-10-07-12-22-48.gh-issue-158981.daAmOF.rst

@@ -0,0 +1,5 @@
+Fix :exc:`TypeError` when subscripting a generic alias such as
+``typing.Dict[T, D]`` and omitting a :class:`~typing.TypeVar` that has a
+default. Also fix the default of a :class:`~typing.TypeVarTuple` being
+inserted as a nested list when subscripting a :class:`types.GenericAlias`
+such as ``tuple[bool, *Ts][()]``.