Assert with walrus operator: Black produced invalid code on pass 1

[alessandroberlati]

Describe the bug

Black produced invalid code on pass 1 when parsing an assert with walrus operator.

To Reproduce

1. create a file test.py

def a():
    assert (x := 1 == 1)

2. Run black test.py

Expected behavior

The output file should be the same as the input file.

Environment

• Version: master/21.4b0
• OS and Python version: macOS Big Sur 11.2.3 and Python3.8

Does this bug also happen on master?

Yes

Additional context

This is the log output:

❯ poetry run black test.py

error: cannot format test.py: INTERNAL ERROR: Black produced invalid code on pass 1: The requested source code has invalid Python 3 syntax.
If you are trying to format Python 2 files please reinstall Black with the 'python2' extra: `python3 -m pip install black[python2]`.. Please report a bug on https://github.com/psf/black/issues.  This invalid output might be helpful: /var/folders/xg/0x59hpln7h70f114ljbbjq400000gp/T/blk_es7z7u5g.log
Oh no! 💥 💔 💥
1 file failed to reformat.

❯ cat /var/folders/xg/0x59hpln7h70f114ljbbjq400000gp/T/blk_es7z7u5g.log

  File "/Users/alessandroberlati/aberlathy/test_black/.venv/lib/python3.8/site-packages/black/__init__.py", line 6508, in assert_equivalent
    dst_ast = parse_ast(dst)
  File "/Users/alessandroberlati/aberlathy/test_black/.venv/lib/python3.8/site-packages/black/__init__.py", line 6404, in parse_ast
    raise SyntaxError(
def a():
    assert x := 1 == 1

It's removing the parenthesis after the assert but that's not valid python.

[JelleZijlstra]

Fixed by #2143