Fix crash in return annotation with walrus

CHANGES.md

@@ -15,6 +15,7 @@
 <!-- Changes that affect Black's preview style -->
 
 - Fix a crash in preview style with assert + parenthesized string (#3415)
+- Fix a crash in preview style with a walrus operator in a return annotation (#3422)
 
 ### Configuration
 

src/black/linegen.py

@@ -1160,6 +1160,7 @@ def maybe_make_parens_invisible_in_atom(
             syms.expr_stmt,
             syms.assert_stmt,
             syms.return_stmt,
+            syms.funcdef,
             # these ones aren't useful to end users, but they do please fuzzers
             syms.for_stmt,
             syms.del_stmt,

tests/data/preview_38/return_annotation_brackets_py38.py

@@ -0,0 +1,9 @@
+# Should not remove parens around walrus operator
+def walrus() -> (x := 1):
+    pass
+
+
+# output
+# Should not remove parens around walrus operator
+def walrus() -> (x := 1):
+    pass

tests/test_format.py

@@ -44,6 +44,13 @@ def test_preview_format(filename: str) -> None:
     )
 
 
+@pytest.mark.parametrize("filename", all_data_cases("preview_38"))
+def test_preview_minimum_python_38_format(filename: str) -> None:
+    source, expected = read_data("preview_38", filename)
+    mode = black.Mode(preview=True)
+    assert_format(source, expected, mode, minimum_version=(3, 8))
+
+
 @pytest.mark.parametrize("filename", all_data_cases("preview_39"))
 def test_preview_minimum_python_39_format(filename: str) -> None:
     source, expected = read_data("preview_39", filename)