fix: Don't merge comments while splitting delimiters

Signed-off-by: RedGuy12 <61329810+RedGuy12@users.noreply.github.com>

CHANGES.md

@@ -10,6 +10,8 @@
 
 <!-- Changes that affect Black's stable style -->
 
+- Don't move comments along with delimiters, which could cause crashes (#4248)
+
 ### Preview style
 
 <!-- Changes that affect Black's preview style -->

src/black/linegen.py

@@ -1194,8 +1194,11 @@ def append_to_line(leaf: Leaf) -> Iterator[Line]:
     for leaf_idx, leaf in enumerate(line.leaves):
         yield from append_to_line(leaf)
 
-        for comment_after in line.comments_after(leaf):
-            yield from append_to_line(comment_after)
+        previous_leaf = line.leaves[leaf_idx - 1] if leaf_idx > 0 else None
+        previous_priority = previous_leaf and bt.delimiters.get(id(previous_leaf))
+        if previous_priority != delimiter_priority:
+            for comment_after in line.comments_after(leaf):
+                yield from append_to_line(comment_after)
 
         lowest_depth = min(lowest_depth, leaf.bracket_depth)
         if leaf.bracket_depth == lowest_depth:
@@ -1215,6 +1218,10 @@ def append_to_line(leaf: Leaf) -> Iterator[Line]:
 
         leaf_priority = bt.delimiters.get(id(leaf))
         if leaf_priority == delimiter_priority:
+            if leaf_idx + 1 < len(line.leaves):
+                for comment_after in line.comments_after(line.leaves[leaf_idx + 1]):
+                    yield from append_to_line(comment_after)
+
             yield current_line
 
             current_line = Line(

tests/data/cases/split_delimiter_comments.py

@@ -0,0 +1,18 @@
+a = (
+    1 +  # type: ignore
+    2  # type: ignore
+)
+a = (
+    1  # type: ignore
+    + 2  # type: ignore
+)
+
+# output
+a = (
+    1  # type: ignore
+    + 2  # type: ignore
+)
+a = (
+    1  # type: ignore
+    + 2  # type: ignore
+)