gh-116325: Raise SyntaxError on ForwardRef with empty string arg

[sobolevn]

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    typing.Type['']
    ~~~~~~~~~~~^^^^
  File "/Users/sobolev/Desktop/cpython2/Lib/typing.py", line 389, in inner
    return _caches[func](*args, **kwds)
           ~~~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "/Users/sobolev/Desktop/cpython2/Lib/typing.py", line 1458, in __getitem__
    params = tuple(_type_check(p, msg) for p in params)
             ~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/sobolev/Desktop/cpython2/Lib/typing.py", line 1458, in <genexpr>
    params = tuple(_type_check(p, msg) for p in params)
                   ~~~~~~~~~~~^^^^^^^^
  File "/Users/sobolev/Desktop/cpython2/Lib/typing.py", line 187, in _type_check
    arg = _type_convert(arg, module=module, allow_special_forms=allow_special_forms)
          ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/sobolev/Desktop/cpython2/Lib/typing.py", line 165, in _type_convert
    return ForwardRef(arg, module=module, is_class=allow_special_forms)
           ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/sobolev/Desktop/cpython2/Lib/typing.py", line 890, in __init__
    raise SyntaxError(f"Forward reference must be an expression -- got {arg!r}")
SyntaxError: Forward reference must be an expression -- got ''

• Issue: Unexpected IndexError in typing.List[''] #116325

[AlexWaygood]

Another one where I guess it's borderline between a feature and a bug... I don't have a strong opinion either way on whether it should be backported!

[AlexWaygood]

(Thanks!)

[sobolevn]

I don't have a strong opinion either way on whether it should be backported!

I think that it should be. I categorize this as a bug, because IndexError feels out of place here. TypeError is expected, SyntaxError also makes sense.

3.11 still has this bug: https://github.com/python/cpython/blob/3.11/Lib/typing.py#L873

[miss-islington-app]

Thanks @sobolevn for the PR, and @AlexWaygood for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11, 3.12.
🐍🍒⛏🤖

[bedevere-app]

GH-116347 is a backport of this pull request to the 3.12 branch.

[bedevere-app]

GH-116348 is a backport of this pull request to the 3.11 branch.