From 21334cc36d0897671fc8e74a9d1aef98c4335833 Mon Sep 17 00:00:00 2001
From: sobolevn <mail@sobolevn.me>
Date: Sat, 13 Sep 2025 15:07:48 +0300
Subject: [PATCH] gh-138859: Fix `TypeError` when there is a `ParamSpec`
 substitution with a default

---
 Lib/test/test_typing.py                       | 22 +++++++++++++++++++
 Lib/typing.py                                 |  2 +-
 ...-09-13-15-07-08.gh-issue-138859.74INn1.rst |  1 +
 3 files changed, 24 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2025-09-13-15-07-08.gh-issue-138859.74INn1.rst

diff --git a/Lib/test/test_typing.py b/Lib/test/test_typing.py
index 8238c62f0715f8..de12dfee8f39fa 100644
--- a/Lib/test/test_typing.py
+++ b/Lib/test/test_typing.py
@@ -1172,6 +1172,28 @@ class C(Generic[*Ts]): pass
                             eval(expected_str)
                         )
 
+    def test_paramspec_default_subst(self):
+        # See https://github.com/python/cpython/issues/138859
+
+        P_default = ParamSpec("P_default", default=...)
+        T = TypeVar("T")
+        T_default = TypeVar("T_default", default=object)
+
+        class A(Generic[T, P_default, T_default]): pass
+
+        # Must not raise:
+        ga = A[int]
+        self.assertEqual(ga.__args__, (int, ..., object))
+        self.assertEqual(ga.__parameters__, ())
+
+        ga = A[int, [str, complex]]
+        self.assertEqual(ga.__args__, (int, (str, complex), object))
+        self.assertEqual(ga.__parameters__, ())
+
+        ga = A[int, [str, complex], float]
+        self.assertEqual(ga.__args__, ((int, (str, complex), float)))
+        self.assertEqual(ga.__parameters__, ())
+
 
 class UnpackTests(BaseTestCase):
 
diff --git a/Lib/typing.py b/Lib/typing.py
index babe3c44d9dc55..5b76a5ef3aa6f0 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -1100,7 +1100,7 @@ def _paramspec_prepare_subst(self, alias, args):
     params = alias.__parameters__
     i = params.index(self)
     if i == len(args) and self.has_default():
-        args = [*args, self.__default__]
+        args = (*args, self.__default__)
     if i >= len(args):
         raise TypeError(f"Too few arguments for {alias}")
     # Special case where Z[[int, str, bool]] == Z[int, str, bool] in PEP 612.
diff --git a/Misc/NEWS.d/next/Library/2025-09-13-15-07-08.gh-issue-138859.74INn1.rst b/Misc/NEWS.d/next/Library/2025-09-13-15-07-08.gh-issue-138859.74INn1.rst
new file mode 100644
index 00000000000000..6eb16ba9e11d17
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-09-13-15-07-08.gh-issue-138859.74INn1.rst
@@ -0,0 +1 @@
+Fixes a :exc:`TypeError` when substituting ``ParamSpec`` with a default.