Support py38-style starred expressions in return statement (#1121)

blib2to3/Grammar.txt

@@ -89,7 +89,7 @@ pass_stmt: 'pass'
 flow_stmt: break_stmt | continue_stmt | return_stmt | raise_stmt | yield_stmt
 break_stmt: 'break'
 continue_stmt: 'continue'
-return_stmt: 'return' [testlist]
+return_stmt: 'return' [testlist_star_expr]
 yield_stmt: yield_expr
 raise_stmt: 'raise' [test ['from' test | ',' test [',' test]]]
 import_stmt: import_name | import_from
@@ -212,4 +212,4 @@ testlist1: test (',' test)*
 encoding_decl: NAME
 
 yield_expr: 'yield' [yield_arg]
-yield_arg: 'from' test | testlist
+yield_arg: 'from' test | testlist_star_expr

tests/data/python38.py

@@ -0,0 +1,27 @@
+#!/usr/bin/env python3.8
+
+
+def starred_return():
+    my_list = ["value2", "value3"]
+    return "value1", *my_list
+
+
+def starred_yield():
+    my_list = ["value2", "value3"]
+    yield "value1", *my_list
+
+
+# output
+
+
+#!/usr/bin/env python3.8
+
+
+def starred_return():
+    my_list = ["value2", "value3"]
+    return "value1", *my_list
+
+
+def starred_yield():
+    my_list = ["value2", "value3"]
+    yield "value1", *my_list

tests/test_black.py

@@ -598,6 +598,16 @@ def test_python37(self) -> None:
         # but not on 3.6, because we use async as a reserved keyword
         self.invokeBlack([str(source_path), "--target-version", "py36"], exit_code=123)
 
+    @patch("black.dump_to_file", dump_to_stderr)
+    def test_python38(self) -> None:
+        source, expected = read_data("python38")
+        actual = fs(source)
+        self.assertFormatEqual(expected, actual)
+        major, minor = sys.version_info[:2]
+        if major > 3 or (major == 3 and minor >= 8):
+            black.assert_equivalent(source, actual)
+        black.assert_stable(source, actual, black.FileMode())
+
     @patch("black.dump_to_file", dump_to_stderr)
     def test_fmtonoff(self) -> None:
         source, expected = read_data("fmtonoff")