fix crash in preview mode with --line-length=1 (#4086)

CHANGES.md

@@ -17,6 +17,7 @@
 - Standalone form feed characters at the module level are no longer removed (#4021)
 - Additional cases of immediately nested tuples, lists, and dictionaries are now
   indented less (#4012)
+- Fix crash in preview mode when using a short `--line-length` (#4086)
 
 ### Configuration
 

src/black/linegen.py

@@ -744,7 +744,7 @@ def left_hand_split(
             if leaf.type in OPENING_BRACKETS:
                 matching_bracket = leaf
                 current_leaves = body_leaves
-    if not matching_bracket:
+    if not matching_bracket or not tail_leaves:
         raise CannotSplit("No brackets found")
 
     head = bracket_split_build_line(

tests/data/cases/return_annotation_brackets_crash_line_length_1.py

@@ -0,0 +1,9 @@
+# flags: --preview --minimum-version=3.10 --line-length=1
+
+def foo() -> tuple[int, int,]:
+    ...
+# output
+def foo() -> tuple[
+    int,
+    int,
+]: ...