Fix type params tuple error

CHANGES.md

@@ -21,6 +21,7 @@
 - Fix crash on `await ...` (where `...` is a literal `Ellipsis`) (#4676)
 - Remove support for pre-python 3.7 `await/async` as soft keywords/variable names
   (#4676)
+- Fix crash on parenthesized expression inside a type parameter bound (#4684)
 
 ### Preview style
 

src/black/linegen.py

@@ -787,12 +787,26 @@ def left_hand_split(
         head_leaves: list[Leaf] = []
         current_leaves = head_leaves
         matching_bracket: Optional[Leaf] = None
-        for leaf in line.leaves:
+        depth = 0
+        for index, leaf in enumerate(line.leaves):
+            if index == 2 and leaf.type == token.LSQB:
+                # A [ at index 2 means this is a type param, so start
+                # tracking the depth
+                depth += 1
+            elif depth > 0:
+                if leaf.type == token.LSQB:
+                    depth += 1
+                elif leaf.type == token.RSQB:
+                    depth -= 1
             if (
                 current_leaves is body_leaves
                 and leaf.type in CLOSING_BRACKETS
                 and leaf.opening_bracket is matching_bracket
                 and isinstance(matching_bracket, Leaf)
+                # If the code is still on LPAR and we are inside a type
+                # param, ignore the match since this is searching
+                # for the function arguments
+                and not (leaf_type == token.LPAR and depth > 0)
             ):
                 ensure_visible(leaf)
                 ensure_visible(matching_bracket)

tests/data/cases/type_params.py

@@ -15,6 +15,8 @@ def magic[Trailing, Comma,](): pass
 
 def weird_syntax[T: lambda: 42, U: a or b](): pass
 
+def name_3[name_0: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa if aaaaaaaaaaa else name_3](): pass
+
 # output
 
 
@@ -62,3 +64,9 @@ def magic[
 
 def weird_syntax[T: lambda: 42, U: a or b]():
     pass
+
+
+def name_3[
+    name_0: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa if aaaaaaaaaaa else name_3
+]():
+    pass