misleading SyntaxError (f-string and type alias related)

[15r10nk]

Bug report

Bug description:

The following code produces a misleading syntax error.

lambda name_3=f'{name_4}': {name_3}

type {}[name_4] = {}

output (Python 3.12.0rc2+):

  File "/home/frank/projects/pysource-codegen/bug.py", line 1
    lambda name_3=f'{name_4}': {name_3}
    ^^^^^^^^^^^^^^^^^^^^^^^^^^
SyntaxError: f-string: lambda expressions are not allowed without parentheses

The first line should be valid python syntax and I would expect to get an syntax error for the wrong type alias name.

side note:
This is generated code. I found this issue during my work for the python 3.12 support for pysource-codegen.

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Linked PRs

• gh-109114: Relax the check for invalid lambdas inside f-strings to avoid false positives #109121
• [3.12] gh-109114: Relax the check for invalid lambdas inside f-strings to avoid false positives (GH-109121). #109155

[AlexWaygood]

The first line should be valid python syntax

Cc. @pablogsal

and I would expect to get an syntax error for the wrong type alias name.

Cc. @JelleZijlstra

[JelleZijlstra]

The wrong type alias does produce a syntax error when you put it by itself:

>>> type {}[name_4] = {}
  File "<stdin>", line 1
    type {}[name_4] = {}
         ^
SyntaxError: invalid syntax

I suspect there's some wrong heuristic in the parser where it simply shows the SyntaxError in the wrong place. That's outside my area of expertise though, so I'll leave this to Pablo.

[pablogsal]

The problem is that the parser finds the issue in the error pass with the lambda first, even if is correct: that's why the type alias by itself raises. You can technically put whatever you want there that's invalid