[3.14] Address invalid inputs of TypeAliasType

src/test_typing_extensions.py

@@ -7466,11 +7466,11 @@ def test_type_params_possibilities(self):
             ((T_default, T), f"non-default type parameter {T!r} follows default"),
             ((P_default, P), f"non-default type parameter {P!r} follows default"),
             ((Ts_default, T), f"non-default type parameter {T!r} follows default"),
-
-            # Potentially add invalid inputs, e.g. literals or classes
-            # depends on upstream
+            # Only type params are accepted
             ((1,), "Expected a type param, got 1"),
             ((str,), f"Expected a type param, got {str!r}"),
+            # Unpack backport behaves like TypeVar in some cases
+            ((Unpack[Ts],), f"Expected a type param, got {re.escape(repr(Unpack[Ts]))}"),
         ]
 
         for case in valid_cases:

src/typing_extensions.py

@@ -3611,7 +3611,10 @@ def __init__(self, name: str, value, *, type_params=()):
             default_value_encountered = False
             parameters = []
             for type_param in type_params:
-                if not isinstance(type_param, (TypeVar, TypeVarTuple, ParamSpec)):
+                if (not isinstance(type_param, (TypeVar, TypeVarTuple, ParamSpec))
+                    # The Unpack backport passes aboves check
+                    or _is_unpack(type_param)
+                ):
                     raise TypeError(f"Expected a type param, got {type_param!r}")
                 has_default = (
                     getattr(type_param, '__default__', NoDefault) is not NoDefault