Use TypeAliasType backport for <3.14

Author: Daraan

src/typing_extensions.py

@@ -3528,36 +3528,9 @@ def __ror__(self, other):
                 return typing.Union[other, self]
 
 
-if hasattr(typing, "TypeAliasType") and sys.version_info >= (3, 14):
+if sys.version_info >= (3, 14):
     TypeAliasType = typing.TypeAliasType
-# 3.12-3.13
-elif hasattr(typing, "TypeAliasType"):
-
-    class TypeAliasType:
-
-        def __new__(self, name: str, value, *, type_params=()):
-            default_value_encountered = False
-            for type_param in type_params:
-                if not isinstance(type_param,
-                                  (typing.TypeVar, typing.TypeVarTuple, typing.ParamSpec)
-                ):
-                    raise TypeError(f"Expected a type param, got {type_param!r}")
-                has_default = (
-                        getattr(type_param, '__default__', NoDefault) is not NoDefault
-                    )
-                if default_value_encountered and not has_default:
-                    raise TypeError(f'non-default type parameter {type_param!r}'
-                                    ' follows default type parameter')
-                if has_default:
-                    default_value_encountered = True
-
-            return typing.TypeAliasType(name, value, type_params=type_params)
-
-        def __init_subclass__(cls, *args, **kwargs):
-            raise TypeError(
-                "type 'typing_extensions.TypeAliasType' is not an acceptable base type"
-            )
-
+# 3.8-3.13
 else:
     def _is_unionable(obj):
         """Corresponds to is_unionable() in unionobject.c in CPython."""