From 01b8d3d4095ebdb91d0d39012a517931625c63cb Mon Sep 17 00:00:00 2001
From: "Yilei \"Dolee\" Yang" <yileiyang@google.com>
Date: Thu, 15 Jun 2023 17:08:26 -0700
Subject: [PATCH] Do not add trailing commas to return type annotations using
 PEP 604 unions (#3735)

Fix #3638: Do not add trailing commas to return type annotations using PEP 604 unions.
---
 CHANGES.md                         |  3 +++
 src/black/linegen.py               |  7 +++++++
 tests/data/simple_cases/pep_604.py | 25 +++++++++++++++++++++++++
 3 files changed, 35 insertions(+)
 create mode 100644 tests/data/simple_cases/pep_604.py

diff --git a/CHANGES.md b/CHANGES.md
index 658faad..fd4d911 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -10,6 +10,9 @@
 
 <!-- Changes that affect Black's stable style -->
 
+- Fix a bug where an illegal trailing comma was added to return type annotations using
+  PEP 604 unions (#3735)
+
 ### Preview style
 
 <!-- Changes that affect Black's preview style -->
diff --git a/src/black/linegen.py b/src/black/linegen.py
index 0091cbb..ad21307 100644
--- a/src/black/linegen.py
+++ b/src/black/linegen.py
@@ -918,6 +918,13 @@ def bracket_split_build_line(
                     )
                     if isinstance(node, Node) and isinstance(node.prev_sibling, Leaf)
                 )
+                # Except the false negatives above for PEP 604 unions where we
+                # can't add the comma.
+                and not (
+                    leaves[0].parent
+                    and leaves[0].parent.next_sibling
+                    and leaves[0].parent.next_sibling.type == token.VBAR
+                )
             )
 
             if original.is_import or no_commas:
diff --git a/tests/data/simple_cases/pep_604.py b/tests/data/simple_cases/pep_604.py
new file mode 100644
index 0000000..b68d59d
--- /dev/null
+++ b/tests/data/simple_cases/pep_604.py
@@ -0,0 +1,25 @@
+def some_very_long_name_function() -> my_module.Asdf | my_module.AnotherType | my_module.YetAnotherType | None:
+    pass
+
+
+def some_very_long_name_function() -> my_module.Asdf | my_module.AnotherType | my_module.YetAnotherType | my_module.EvenMoreType | None:
+    pass
+
+
+# output
+
+
+def some_very_long_name_function() -> (
+    my_module.Asdf | my_module.AnotherType | my_module.YetAnotherType | None
+):
+    pass
+
+
+def some_very_long_name_function() -> (
+    my_module.Asdf
+    | my_module.AnotherType
+    | my_module.YetAnotherType
+    | my_module.EvenMoreType
+    | None
+):
+    pass
-- 
2.39.2