Hang when parsing # fmt: on in the middle of an expression

[remyjette]

Describe the bug
black hangs when parsing # fmt: on in the middle of an expression

To Reproduce

1. Take this file

from subprocess import run

path = ["."]
run(
    # fmt: off
    [
        "ls",
        "-la",
    ]
    # fmt: on
    + path,
    check=True,
)

2. Run Black on it with these arguments --target-version py38
3. Observe that black never exits.

Expected behavior
Either black should format the file fine (19.10b0 does not have trouble here) or it should error out and tell me it failed to format.

Environment (please complete the following information):

• Version: [e.g. master] 20.8b1
• OS and Python version: [e.g. Linux/Python 3.7.4rc1] Linux/Python 3.8.5

Does this bug also happen on master? To answer this, you have two options:

1. Use the online formatter at https://black.now.sh/?version=master, which will use the
   latest master branch.
2. Or run Black on your machine:
   • create a new virtualenv (make sure it's the same Python version);
   • clone this repository;
   • run pip install -e .;
   • make sure it's sane by running python -m unittest; and
   • run black like you did last time.

Yes, it is happening on the online formatter as well.

Additional context
If I remove check=True, from my example, it works fine. If I move # fmt: on after + path, it works fine.

We've modified our code to put # fmt: on in a more reasonable location, but still wanted to report it as I wouldn't expect a hang.

[ichard26]

Yeah, this is definitely hanging and wow does this (partial) call graph look wrong. Created using 858225d and the code in the bug report.