Bug: fails to properly format constrained generics with new 3.12 syntax

[dominiquegarmier]

Describe the bug
when formatting the following snippet

def func1[T: (int, str)](a,): ...

we get

def func1[T: (
    int,
    str,
)](a,): ...

we would expect:

def func[T: (int, str)](
    a,
): ...

black version

black --version
black, 25.1.0 (compiled: yes)
Python (CPython) 3.12.7

This is more specific case of a bug discussed in #4254 (which has been fixed in 25.1.0)

[MeGaGiGaGon]

Completely guessing, but the fix for this may be the same as what I did in #4684, just with the bug happening in the trailing comma code, where the linegen gets confused and thinks the parenthesis inside the type params is the function argument parens, splitting early because of it.

crash details that only applies on older main version

On doing some testing, you can use this to cause a crash with the example from #4647 slightly modified playground

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

Crash diff

INTERNAL ERROR: Black 25.1.1.dev28+g314f8cf on Python (CPython) 3.12.9 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_dyx9zhpc.log

--- src
+++ dst
@@ -32,35 +32,41 @@
         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
             name=
             'name_0',  # str
         )  # /TypeVar
     )  # /FunctionDef
     type_ignores=

Though this also causes a crash, so my fix in #4684 might have been too specific and there may be a larger issue at play somewhere different. playground

def name_3[name_0: (a)](a,): pass

Crash diff

INTERNAL ERROR: Black 25.1.1.dev28+g314f8cf on Python (CPython) 3.12.9 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_l198sm6e.log

--- src
+++ dst
@@ -32,17 +32,23 @@
         type_comment=
         None,  # NoneType
         type_params=
         TypeVar(
             bound=
-            Name(
+            Tuple(
                 ctx=
                 Load(
                 )  # /Load
-                id=
-                'a',  # str
-            )  # /Name
+                elts=
+                Name(
+                    ctx=
+                    Load(
+                    )  # /Load
+                    id=
+                    'a',  # str
+                )  # /Name
+            )  # /Tuple
             name=
             'name_0',  # str
         )  # /TypeVar
     )  # /FunctionDef
     type_ignores=

[MeGaGiGaGon]

On further testing, it looks like the playground's main is out-of-sync with current main for some reason. It looks like #4684 did work for fixing the crashes, but the issue is still present, giving this on main:

def func1[
    T: (int, str)
](a,): ...

So the issue is probably further up the tree from the changes I made in #4684, since it did stop the inner tuple from being expanded, but it looks like the comma is still incorrectly expanding the type param.

[aviralgarg05]

I would like to solve the issue for hacktoberfest

[MeGaGiGaGon]

@aviralgarg05 Feel free to give it a try. See my above posts for more info on what I suspect is going wrong, and I'd be happy to review any PRs you open for this issue (or any other)

[Exgene]

I needed some help with test cases and further testing, If you could help it would be amazing @MeGaGiGaGon