gh-96385: Correctly raise error on [*T, *V] substitution (GH-96386) (#96407)

(cherry picked from commit 7517735)

Co-authored-by: Nikita Sobolev <[email protected]>

Co-authored-by: Nikita Sobolev <[email protected]>

Author: miss-islington

Lib/test/test_typing.py

@@ -591,6 +591,7 @@ class GenericAliasSubstitutionTests(BaseTestCase):
     def test_one_parameter(self):
         T = TypeVar('T')
         Ts = TypeVarTuple('Ts')
+        Ts2 = TypeVarTuple('Ts2')
 
         class C(Generic[T]): pass
 
@@ -616,6 +617,8 @@ class C(Generic[T]): pass
             # Should definitely raise TypeError: list only takes one argument.
             ('list[T, *tuple_type[int, ...]]',    '[int]',                   'list[int, *tuple_type[int, ...]]'),
             ('List[T, *tuple_type[int, ...]]',    '[int]',                   'TypeError'),
+            # Should raise, because more than one `TypeVarTuple` is not supported.
+            ('generic[*Ts, *Ts2]',                '[int]',                   'TypeError'),
         ]
 
         for alias_template, args_template, expected_template in tests:

Lib/typing.py

@@ -1071,7 +1071,7 @@ def __typing_subst__(self, arg):
     def __typing_prepare_subst__(self, alias, args):
         params = alias.__parameters__
         typevartuple_index = params.index(self)
-        for param in enumerate(params[typevartuple_index + 1:]):
+        for param in params[typevartuple_index + 1:]:
             if isinstance(param, TypeVarTuple):
                 raise TypeError(f"More than one TypeVarTuple parameter in {alias}")
 

Misc/NEWS.d/next/Library/2022-08-29-15-28-39.gh-issue-96385.uLRTsf.rst

@@ -0,0 +1,3 @@
+Fix ``TypeVarTuple.__typing_prepare_subst__``. ``TypeError`` was not raised
+when using more than one ``TypeVarTuple``, like ``[*T, *V]`` in type alias
+substitutions.