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

3 files changed, 7 insertions, 1 deletions

diff --git a/Lib/test/test_typing.py b/Lib/test/test_typing.py
index ca7bd8e..7eea019 100644
--- a/Lib/test/test_typing.py
+++ b/Lib/test/test_typing.py
@@ -596,6 +596,7 @@ class GenericAliasSubstitutionTests(BaseTestCase):
     def test_one_parameter(self):
         T = TypeVar('T')
         Ts = TypeVarTuple('Ts')
+        Ts2 = TypeVarTuple('Ts2')
 
         class C(Generic[T]): pass
 
@@ -621,6 +622,8 @@ class GenericAliasSubstitutionTests(BaseTestCase):
             # 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:
diff --git a/Lib/typing.py b/Lib/typing.py
index 43ef07e..596744e 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -1074,7 +1074,7 @@ class TypeVarTuple(_Final, _Immutable, _PickleUsingNameMixin, _root=True):
     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}")
 
diff --git a/Misc/NEWS.d/next/Library/2022-08-29-15-28-39.gh-issue-96385.uLRTsf.rst b/Misc/NEWS.d/next/Library/2022-08-29-15-28-39.gh-issue-96385.uLRTsf.rst
new file mode 100644
index 0000000..5735482
--- /dev/null
+++ b/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.