According to the documentation, a lambda expression at the `comp_if` position of a comprehension is allowed (can be parsed as `lambda_expr_nocond`). But this seems broken in Python 3.9 PEG parser. Example:

user@host:/$ python3.8
Python 3.8.9 (default, Apr  3 2021, 01:00:00)
[GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> [x for x in range(10) if lambda: 1]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>>
user@host:/$ python3.9
Python 3.9.3 (default, Apr  3 2021, 00:51:37)
[GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> [x for x in range(10) if lambda: 1]
  File "<stdin>", line 1
    [x for x in range(10) if lambda: 1]
                             ^
SyntaxError: invalid syntax

This looks like a duplicate of https://bugs.python.org/issue41848

OK I didn't find this duplicate issue. After reading that, I think we should update the documentation (https://docs.python.org/3/reference/expressions.html). `comp_if       ::=  "if" expression_nocond [comp_iter]` can become `comp_if       ::=  "if" or_test [comp_iter]`, and all `nocond` stuffs can be removed.

Oh, nice improvement suggestion for the docs! Do you feel comfortable submitting a small PR for that?

I'll work on it soon.

New changeset 0fdf11e8e901a5f47149232557a7f9726b8177c9 by Saiyang Gou in branch 'master':
bpo-43755: Update docs to reflect that lambda is not allowed in `comp_if` since 3.9 (GH-25231)
https://github.com/python/cpython/commit/0fdf11e8e901a5f47149232557a7f9726b8177c9

Thanks for the quick fix, Saiyang Gou

New changeset f91fc7a679e076cb9a703f6db4d95b63935562e3 by Miss Islington (bot) in branch '3.9':
bpo-43755: Update docs to reflect that lambda is not allowed in `comp_if` since 3.9 (GH-25231) (GH-25233)
https://github.com/python/cpython/commit/f91fc7a679e076cb9a703f6db4d95b63935562e3