[3.14] Address invalid inputs of TypeAliasType (#477)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

Author: Daraan

CHANGELOG.md

@@ -23,6 +23,9 @@ aliases that have a `Concatenate` special form as their argument.
   `Ellipsis` as an argument. Patch by [Daraan](https://github.com/Daraan).
 - Fix error in subscription of `Unpack` aliases causing nested Unpacks 
   to not be resolved correctly. Patch by [Daraan](https://github.com/Daraan).
+- Backport of CPython PR [#124795](https://github.com/python/cpython/pull/124795)
+  and fix that `TypeAliasType` not raising an error on non-tupple inputs for `type_params`.
+  Patch by [Daraan](https://github.com/Daraan).
 
 # Release 4.12.2 (June 7, 2024)
 

src/test_typing_extensions.py

@@ -6192,6 +6192,10 @@ def test_typing_extensions_defers_when_possible(self):
                 'AsyncGenerator', 'ContextManager', 'AsyncContextManager',
                 'ParamSpec', 'TypeVar', 'TypeVarTuple', 'get_type_hints',
             }
+        if sys.version_info < (3, 14):
+            exclude |= {
+                'TypeAliasType'
+            }
         if not typing_extensions._PEP_728_IMPLEMENTED:
             exclude |= {'TypedDict', 'is_typeddict'}
         for item in typing_extensions.__all__:
@@ -7402,6 +7406,80 @@ def test_no_instance_subclassing(self):
             class MyAlias(TypeAliasType):
                 pass
 
+    def test_type_var_compatibility(self):
+        # Regression test to assure compatibility with typing variants
+        typingT = typing.TypeVar('typingT')
+        T1 = TypeAliasType("TypingTypeVar", ..., type_params=(typingT,))
+        self.assertEqual(T1.__type_params__, (typingT,))
+
+        # Test typing_extensions backports
+        textT = TypeVar('textT')
+        T2 = TypeAliasType("TypingExtTypeVar", ..., type_params=(textT,))
+        self.assertEqual(T2.__type_params__, (textT,))
+
+        textP = ParamSpec("textP")
+        T3 = TypeAliasType("TypingExtParamSpec", ..., type_params=(textP,))
+        self.assertEqual(T3.__type_params__, (textP,))
+
+        textTs = TypeVarTuple("textTs")
+        T4 = TypeAliasType("TypingExtTypeVarTuple", ..., type_params=(textTs,))
+        self.assertEqual(T4.__type_params__, (textTs,))
+
+    @skipUnless(TYPING_3_10_0, "typing.ParamSpec is not available before 3.10")
+    def test_param_spec_compatibility(self):
+        # Regression test to assure compatibility with typing variant
+        typingP = typing.ParamSpec("typingP")
+        T5 = TypeAliasType("TypingParamSpec", ..., type_params=(typingP,))
+        self.assertEqual(T5.__type_params__, (typingP,))
+
+    @skipUnless(TYPING_3_12_0, "typing.TypeVarTuple is not available before 3.12")
+    def test_type_var_tuple_compatibility(self):
+        # Regression test to assure compatibility with typing variant
+        typingTs = typing.TypeVarTuple("typingTs")
+        T6 = TypeAliasType("TypingTypeVarTuple", ..., type_params=(typingTs,))
+        self.assertEqual(T6.__type_params__, (typingTs,))
+
+    def test_type_params_possibilities(self):
+        T = TypeVar('T')
+        # Test not a tuple
+        with self.assertRaisesRegex(TypeError, "type_params must be a tuple"):
+            TypeAliasType("InvalidTypeParams", List[T], type_params=[T])
+
+        # Test default order and other invalid inputs
+        T_default = TypeVar('T_default', default=int)
+        Ts = TypeVarTuple('Ts')
+        Ts_default = TypeVarTuple('Ts_default', default=Unpack[Tuple[str, int]])
+        P = ParamSpec('P')
+        P_default = ParamSpec('P_default', default=[str, int])
+
+        # NOTE: PEP 696 states: "TypeVars with defaults cannot immediately follow TypeVarTuples"
+        # this is currently not enforced for the type statement and is not tested.
+        # PEP 695: Double usage of the same name is also not enforced and not tested.
+        valid_cases = [
+            (T, P, Ts),
+            (T, Ts_default),
+            (P_default, T_default),
+            (P, T_default, Ts_default),
+            (T_default, P_default, Ts_default),
+        ]
+        invalid_cases = [
+            ((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"),
+            # Only type params are accepted
+            ((1,), "Expected a type param, got 1"),
+            ((str,), f"Expected a type param, got {str!r}"),
+            # Unpack is not a TypeVar but isinstance(Unpack[Ts], TypeVar) is True in Python < 3.12
+            ((Unpack[Ts],), f"Expected a type param, got {re.escape(repr(Unpack[Ts]))}"),
+        ]
+
+        for case in valid_cases:
+            with self.subTest(type_params=case):
+                TypeAliasType("OkCase", List[T], type_params=case)
+        for case, msg in invalid_cases:
+            with self.subTest(type_params=case):
+                with self.assertRaisesRegex(TypeError, msg):
+                    TypeAliasType("InvalidCase", List[T], type_params=case)
 
 class DocTests(BaseTestCase):
     def test_annotation(self):

src/typing_extensions.py

@@ -3528,8 +3528,9 @@ def __ror__(self, other):
                 return typing.Union[other, self]
 
 
-if hasattr(typing, "TypeAliasType"):
+if sys.version_info >= (3, 14):
     TypeAliasType = typing.TypeAliasType
+# 3.8-3.13
 else:
     def _is_unionable(obj):
         """Corresponds to is_unionable() in unionobject.c in CPython."""
@@ -3602,11 +3603,29 @@ class TypeAliasType:
         def __init__(self, name: str, value, *, type_params=()):
             if not isinstance(name, str):
                 raise TypeError("TypeAliasType name must be a string")
+            if not isinstance(type_params, tuple):
+                raise TypeError("type_params must be a tuple")
             self.__value__ = value
             self.__type_params__ = type_params
 
+            default_value_encountered = False
             parameters = []
             for type_param in type_params:
+                if (
+                    not isinstance(type_param, (TypeVar, TypeVarTuple, ParamSpec))
+                    # 3.8-3.11
+                    # Unpack Backport passes isinstance(type_param, TypeVar)
+                    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
+                )
+                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
                 if isinstance(type_param, TypeVarTuple):
                     parameters.extend(type_param)
                 else: