Fix ParamSpec ellipsis default for <3.10 (#279)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

CHANGELOG.md

@@ -5,6 +5,8 @@
   called on a concrete subclass of a generic class. Patch by Alex Waygood
   (backporting https://github.com/python/cpython/pull/107584, by James
   Hilton-Balfe).
+- Fix bug where `ParamSpec(default=...)` would raise a `TypeError` on Python
+  versions <3.11. Patch by James Hilton-Balfe
 
 # Release 4.7.1 (July 2, 2023)
 

doc/index.rst

@@ -278,6 +278,11 @@ Special typing primitives
 
       The implementation was changed for compatibility with Python 3.12.
 
+   .. versionchanged:: 4.8.0
+
+      Passing an ellipsis literal (``...``) to *default* now works on Python
+      3.10 and lower.
+
 .. class:: ParamSpecArgs
 
 .. class:: ParamSpecKwargs

src/test_typing_extensions.py

@@ -5593,6 +5593,9 @@ def test_paramspec(self):
         class A(Generic[P]): ...
         Alias = typing.Callable[P, None]
 
+        P_default = ParamSpec('P_default', default=...)
+        self.assertIs(P_default.__default__, ...)
+
     def test_typevartuple(self):
         Ts = TypeVarTuple('Ts', default=Unpack[Tuple[str, int]])
         self.assertEqual(Ts.__default__, Unpack[Tuple[str, int]])

src/typing_extensions.py

@@ -1277,7 +1277,10 @@ def _set_default(type_param, default):
         type_param.__default__ = tuple((typing._type_check(d, "Default must be a type")
                                         for d in default))
     elif default != _marker:
-        type_param.__default__ = typing._type_check(default, "Default must be a type")
+        if isinstance(type_param, ParamSpec) and default is ...:  # ... not valid <3.11
+            type_param.__default__ = default
+        else:
+            type_param.__default__ = typing._type_check(default, "Default must be a type")
     else:
         type_param.__default__ = None