Internal error in the formatting of the if expression within a type annotation

[15r10nk]

Describe the bug

The following code can not be parsed/formatted by black:

def name_3[name_0: [] if name_2 else name_3]():
    pass

(playground)

black reported the following error:

> black -l 46 -t py313 bug.py
error: cannot format bug.py: INTERNAL ERROR: Black 25.1.1.dev21+g944a38e.d20250317 on Python (CPython) 3.13.1 produced code that is not equivalent to the source.  Please report a bug on https://github.com/psf/black/issues.  This diff might be helpful: /tmp/blk_0ajkvzbr.log

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

the reported diff in /tmp/blk_0ajkvzbr.log is:

--- src
+++ dst
@@ -24,35 +24,41 @@
         type_comment=
         None,  # NoneType
         type_params=
         TypeVar(
             bound=
-            IfExp(
-                body=
-                List(
-                    ctx=
-                    Load(
-                    )  # /Load
-                    elts=
-                )  # /List
-                orelse=
-                Name(
-                    ctx=
-                    Load(
-                    )  # /Load
-                    id=
-                    'name_3',  # str
-                )  # /Name
-                test=
-                Name(
-                    ctx=
-                    Load(
-                    )  # /Load
-                    id=
-                    'name_2',  # str
-                )  # /Name
-            )  # /IfExp
+            Tuple(
+                ctx=
+                Load(
+                )  # /Load
+                elts=
+                IfExp(
+                    body=
+                    List(
+                        ctx=
+                        Load(
+                        )  # /Load
+                        elts=
+                    )  # /List
+                    orelse=
+                    Name(
+                        ctx=
+                        Load(
+                        )  # /Load
+                        id=
+                        'name_3',  # str
+                    )  # /Name
+                    test=
+                    Name(
+                        ctx=
+                        Load(
+                        )  # /Load
+                        id=
+                        'name_2',  # str
+                    )  # /Name
+                )  # /IfExp
+            )  # /Tuple
             default_value=
             None,  # NoneType
             name=
             'name_0',  # str
         )  # /TypeVar

but it can be parsed by cpython:

from ast import parse
parse(
    'def name_3[name_0: [] if name_2 else name_3]():\n'
    '    pass\n'
)

The code can be formatted with black -l 46 -t py313 bug.py --fast:

def name_3[name_0: (
    [] if name_2 else name_3,
)]():
    pass

Environment

• Black's version: current main (d0ff3bd)
• OS and Python version: Linux/Python 3.13.1 (main, Jan 14 2025, 22:47:38) [Clang 19.1.6 ]

Additional context

The bug was found by pysource-codegen (see #3908)
The above problem description was created from a script,
let me know if you think it can be improved.

[Zeecoworld]

yes it is, can you explain better what you driving at pls?

[15r10nk]

I'm the author of pysource-codegen and used it to do some fuzz testing with black (see #3908).

I wrote a small script which searches for bugs like the one above and generates this report. The goal is to integrate these tests in the CI of black and to be able to find issues before they are found by some user.

Cases like the above are real edge cases which are unlikely to be a problem in the real world, but you can not wish to find the important one without fixing the edge cases too.

[Zeecoworld]

I am interested in working on this. Can I make a PR ??