Cover more edge cases

src/black/linegen.py

@@ -575,10 +575,19 @@ def bracket_split_build_line(
                 and opening_bracket.value == "("
                 and not any(leaf.type == token.COMMA for leaf in leaves)
                 # In particular, don't add one within a parenthesized return annotation.
-                and not (
-                    leaves[0].parent
-                    and leaves[0].parent.prev_sibling
-                    and leaves[0].parent.prev_sibling.type == RARROW
+                # Unfortunately the indicator we're in a return annotation (RARROW) may
+                # be defined directly in the parent node OR the *parent of the parent*
+                # depending on how complex the return annotation is. Oh yeah and we
+                # have to be careful to not trigger AttributeError making this unpretty.
+                # This isn't perfect and there's some false negatives but they are in
+                # contexts were a comma is actually fine.
+                and not any(
+                    node.prev_sibling.type == RARROW
+                    for node in (
+                        leaves[0].parent,
+                        getattr(leaves[0].parent, "parent", None),
+                    )
+                    if isinstance(node, Node) and isinstance(node.prev_sibling, Leaf)
                 )
             )
 

tests/data/function_trailing_comma.py

@@ -37,6 +37,24 @@ def some_method_with_a_really_long_name(very_long_parameter_so_yeah: str, anothe
     pass
 
 
+def func() -> (
+    also_super_long_type_annotation_that_may_cause_an_AST_related_crash_in_black(this_shouldn_t_get_a_trailing_comma_too)
+):
+    pass
+
+
+def func() -> ((also_super_long_type_annotation_that_may_cause_an_AST_related_crash_in_black(
+        this_shouldn_t_get_a_trailing_comma_too
+    ))
+):
+    pass
+
+
+def func() -> ( a:= also_super_long_type_annotation_that_may_cause_an_AST_related_crash_in_black(
+        this_shouldn_t_get_a_trailing_comma_too
+    )):
+    pass
+
 # output
 
 def f(
@@ -118,3 +136,29 @@ def some_method_with_a_really_long_name(
     another_case_of_returning_a_deeply_nested_import_of_a_type_i_suppose_cause_why_not
 ):
     pass
+
+
+def func() -> (
+    also_super_long_type_annotation_that_may_cause_an_AST_related_crash_in_black(
+        this_shouldn_t_get_a_trailing_comma_too
+    )
+):
+    pass
+
+
+def func() -> (
+    (
+        also_super_long_type_annotation_that_may_cause_an_AST_related_crash_in_black(
+            this_shouldn_t_get_a_trailing_comma_too
+        )
+    )
+):
+    pass
+
+
+def func() -> (
+    a := also_super_long_type_annotation_that_may_cause_an_AST_related_crash_in_black(
+        this_shouldn_t_get_a_trailing_comma_too
+    )
+):
+    pass