[3.12] gh-116325: Raise `SyntaxError` rather than `IndexError` on ForwardRef with empty string arg (GH-116341) (#116347)

gh-116325: Raise `SyntaxError` rather than `IndexError` on ForwardRef with empty string arg (GH-116341)
(cherry picked from commit a29998a06bf75264c3faaeeec4584a5f75b45a1f)

Co-authored-by: Nikita Sobolev <mail@sobolevn.me>

2 files changed, 7 insertions, 1 deletions

diff --git a/Lib/test/test_typing.py b/Lib/test/test_typing.py
index e0f7146..8ad26a7 100644
--- a/Lib/test/test_typing.py
+++ b/Lib/test/test_typing.py
@@ -5655,6 +5655,12 @@ class ForwardRefTests(BaseTestCase):
         with self.assertRaises(SyntaxError):
             get_type_hints(foo)
 
+    def test_syntax_error_empty_string(self):
+        for form in [typing.List, typing.Set, typing.Type, typing.Deque]:
+            with self.subTest(form=form):
+                with self.assertRaises(SyntaxError):
+                    form['']
+
     def test_name_error(self):
 
         def foo(a: 'Noode[T]'):
diff --git a/Lib/typing.py b/Lib/typing.py
index 7581c16..1d8feed 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -886,7 +886,7 @@ class ForwardRef(_Final, _root=True):
         # If we do `def f(*args: *Ts)`, then we'll have `arg = '*Ts'`.
         # Unfortunately, this isn't a valid expression on its own, so we
         # do the unpacking manually.
-        if arg[0] == '*':
+        if arg.startswith('*'):
             arg_to_compile = f'({arg},)[0]'  # E.g. (*Ts,)[0] or (*tuple[int, int],)[0]
         else:
             arg_to_compile = arg