Let black run on files even if ast.parse with type_comments=True fails

[tushar-deepsource]

Is your feature request related to a problem? Please describe.

Right now, if a file parsing because of invalid type comments (even if the code runs just fine), black won't format the file.

For example:

import typing

def foo(
    # type: typing.List
    bar,
):
    print(bar)

foo(10)

While the code still runs:

$ python3 asd.py            
10

Black no longer formats it:

$ black asd.py        
error: cannot format asd.py: cannot use --safe with this file; failed to parse source file AST: invalid syntax (<unknown>, line 4)
This could be caused by running Black with an older Python version that does not support new syntax used in your source file.

Oh no! 💥 💔 💥
1 file failed to reformat.

This is because ast.parse on that code fails if you pass type_comments=True:

>>> import ast
>>> ast.parse(open('asd.py').read(), type_comments=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/homebrew/Cellar/[email protected]/3.10.9/Frameworks/Python.framework/Versions/3.10/lib/python3.10/ast.py", line 50, in parse
    return compile(source, filename, mode, flags,
  File "<unknown>", line 4
    # type: typing.List
            ^^^^^^^^^^^
SyntaxError: invalid syntax

(Black does work if you do black asd.py --fast, as it no longer does the AST based check.)

Describe the solution you'd like

If ast.parse(..., type_comments=True) fails for the before/after check, maybe doing it with type_comments=False is fine, just for that file?

Describe alternatives you've considered

None so far.

Additional context

I don't think this can be considered an upstream bug, as this is indeed invalid syntax for a type checker's perspective. But black shouldn't care if the types are correct, in any way shape or form.

[tushar-deepsource]

If the change is approved, I can work on the PR as well.

[JelleZijlstra]

Makes sense to me. We should try ast.parse(type_comments=False) only as a fallback, so performance won't be impacted for the case where type_comments=True parses correctly.