Unexpected IndexError in typing.List['']

[jureslak]

Bug report

Bug description:

Calling typing.List[''] produces an unexpected IndexError.

>>> import typing
>>> typing.List['']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.11/typing.py", line 362, in inner
    return cached(*args, **kwds)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/typing.py", line 1575, in __getitem__
    params = tuple(_type_check(p, msg) for p in params)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/typing.py", line 1575, in <genexpr>
    params = tuple(_type_check(p, msg) for p in params)
                   ^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/typing.py", line 186, in _type_check
    arg = _type_convert(arg, module=module, allow_special_forms=allow_special_forms)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/typing.py", line 164, in _type_convert
    return ForwardRef(arg, module=module, is_class=allow_special_forms)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/typing.py", line 859, in __init__
    if arg[0] == '*':
       ~~~^^^
IndexError: string index out of range

The construct is invalid, but I'd expect a more friendly error, something similar to

>>> typing.Dict[1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.11/typing.py", line 365, in inner
    return func(*args, **kwds)
           ^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/typing.py", line 1576, in __getitem__
    _check_generic(self, params, self._nparams)
  File "/usr/lib/python3.11/typing.py", line 293, in _check_generic
    raise TypeError(f"Too {'many' if alen > elen else 'few'} arguments for {cls};"
TypeError: Too few arguments for typing.Dict; actual 1, expected 2

CPython versions tested on:

3.11

Operating systems tested on:

Linux

Linked PRs

• gh-116325: Raise SyntaxError on ForwardRef with empty string arg #116341
• [3.12] gh-116325: Raise SyntaxError rather than IndexError on ForwardRef with empty string arg (GH-116341) #116347
• [3.11] gh-116325: Raise SyntaxError rather than IndexError on ForwardRef with empty string arg (GH-116341) #116348

[AlexWaygood]

Thanks for the report @jureslak, and thanks for fixing @sobolevn!