Make detection of TypeVar defaults robust to the CPython PEP-696 impl…

…ementation

src/cattrs/gen/_generics.py

@@ -5,6 +5,30 @@
 from .._compat import get_args, get_origin, is_generic
 
 
+def _tvar_has_default(tvar) -> bool:
+    """Does `tvar` have a default?
+
+    In CPython 3.13+ and typing_extensions>=4.12.0:
+    - TypeVars have a `no_default()` method for detecting
+      if a TypeVar has a default
+    - TypeVars with `default=None` have `__default__` set to `None`
+    - TypeVars with no `default` parameter passed
+      have `__default__` set to `typing(_extensions).NoDefault
+
+    On typing_exensions<4.12.0:
+    - TypeVars do not have a `no_default()` method for detecting
+      if a TypeVar has a default
+    - TypeVars with `default=None` have `__default__` set to `NoneType`
+    - TypeVars with no `default` parameter passed
+      have `__default__` set to `typing(_extensions).NoDefault
+    """
+    try:
+        return tvar.has_default()
+    except AttributeError:
+        # compatibility for typing_extensions<4.12.0
+        return getattr(tvar, "__default__", None) is not None
+
+
 def generate_mapping(cl: type, old_mapping: dict[str, type] = {}) -> dict[str, type]:
     """Generate a mapping of typevars to actual types for a generic class."""
     mapping = dict(old_mapping)
@@ -35,20 +59,15 @@ def generate_mapping(cl: type, old_mapping: dict[str, type] = {}) -> dict[str, t
             base_args = base.__args__
             if hasattr(base.__origin__, "__parameters__"):
                 base_params = base.__origin__.__parameters__
-            elif any(
-                getattr(base_arg, "__default__", None) is not None
-                for base_arg in base_args
-            ):
+            elif any(_tvar_has_default(base_arg) for base_arg in base_args):
                 # TypeVar with a default e.g. PEP 696
                 # https://www.python.org/dev/peps/pep-0696/
                 # Extract the defaults for the TypeVars and insert
                 # them into the mapping
                 mapping_params = [
                     (base_arg, base_arg.__default__)
                     for base_arg in base_args
-                    # Note: None means no default was provided, since
-                    # TypeVar("T", default=None) sets NoneType as the default
-                    if getattr(base_arg, "__default__", None) is not None
+                    if _tvar_has_default(base_arg)
                 ]
                 base_params, base_args = zip(*mapping_params)
             else: