Support default type parameters in Generic

src/test_typing_extensions.py

@@ -7815,6 +7815,14 @@ def test_typevar(self):
         class A(Generic[T]): ...
         self.assertEqual(Optional[T].__args__, (T, type(None)))
 
+    def test_generic_typevar_default(self):
+        R = TypeVar("R")
+        T = TypeVar("T", default=int)
+
+        class A(Generic[R, T]): ...
+
+        self.assertEqual(get_args(A[str]), (str, int))
+
     def test_typevar_none(self):
         U = typing_extensions.TypeVar('U')
         U_None = typing_extensions.TypeVar('U_None', default=None)

src/typing_extensions.py

@@ -4418,5 +4418,71 @@ def type_repr(value):
 # These are defined unconditionally because they are used in
 # typing-extensions itself.
 Generic = typing.Generic
+
+# Python 3.9-3.12 do not ask type parameters to fill in their defaults when
+# specializing a generic alias.  TypeVarLike implementations above expose the
+# same substitution hook used by newer versions of typing, so apply it before
+# delegating to the standard library alias implementation.
+if not _PEP_696_IMPLEMENTED:
+    _generic_alias_getitem = typing._GenericAlias.__getitem__
+
+    @typing._tp_cache
+    def _generic_alias_getitem_with_defaults(self, params):
+        if not isinstance(params, tuple):
+            params = (params,)
+        params = tuple(typing._type_convert(param) for param in params)
+
+        for param in self.__parameters__:
+            prepare = getattr(param, "__typing_prepare_subst__", None)
+            if prepare is None:
+                continue
+            if type(param).__name__ == "TypeVar":
+                if not getattr(param, "has_default", lambda: False)():
+                    continue
+            elif not (
+                sys.version_info < (3, 11)
+                and type(param).__name__ == "TypeVarTuple"
+            ):
+                continue
+            params = prepare(self, params)
+
+        return _generic_alias_getitem(self, params)
+
+    typing._GenericAlias.__getitem__ = _generic_alias_getitem_with_defaults
+
+    _generic_class_getitem_descriptor = typing.Generic.__dict__["__class_getitem__"]
+    if hasattr(_generic_class_getitem_descriptor, "__func__"):
+        _generic_class_getitem = _generic_class_getitem_descriptor.__func__
+    else:
+        # Python 3.12 stores this as a classmethod descriptor rather than a
+        # classmethod object.  Bind it through the subclass so the original
+        # implementation receives the class being specialized.
+        def _generic_class_getitem(cls, params):
+            return _generic_class_getitem_descriptor.__get__(None, cls)(params)
+
+    @typing._tp_cache
+    def _generic_class_getitem_with_defaults(cls, params):
+        if not isinstance(params, tuple):
+            params = (params,)
+        params = tuple(typing._type_convert(param) for param in params)
+
+        for param in getattr(cls, "__parameters__", ()):
+            prepare = getattr(param, "__typing_prepare_subst__", None)
+            if prepare is None:
+                continue
+            if type(param).__name__ == "TypeVar":
+                if not getattr(param, "has_default", lambda: False)():
+                    continue
+            elif not (
+                sys.version_info < (3, 11)
+                and type(param).__name__ == "TypeVarTuple"
+            ):
+                continue
+            params = prepare(cls, params)
+
+        return _generic_class_getitem(cls, params)
+
+    typing.Generic.__class_getitem__ = classmethod(_generic_class_getitem_with_defaults)
+
 ForwardRef = typing.ForwardRef
 Annotated = typing.Annotated