grammar: accept open sequences on match subject (GH-2639)

* grammar: accept open sequences on match subject
* give an example about the fixed match subject

CHANGES.md

@@ -5,6 +5,7 @@
 ### _Black_
 
 - Fixed Python 3.10 support on platforms without ProcessPoolExecutor (#2631)
+- Fixed `match` statements with open sequence subjects, like `match a, b:` (#2639)
 
 ## 21.11b1
 

src/blib2to3/Grammar.txt

@@ -238,7 +238,7 @@ yield_arg: 'from' test | testlist_star_expr
 # to reformat them.
 
 match_stmt: "match" subject_expr ':' NEWLINE INDENT case_block+ DEDENT
-subject_expr: namedexpr_test
+subject_expr: namedexpr_test (',' namedexpr_test)* [',']
 
 # cases
 case_block: "case" patterns [guard] ':' suite

tests/data/pattern_matching_extras.py

@@ -27,3 +27,19 @@ def func(match: case, case: match) -> case:
             ...
         case func(match, case):
             ...
+
+
+match maybe, multiple:
+    case perhaps, 5:
+        pass
+    case perhaps, 6,:
+        pass
+
+
+match more := (than, one), indeed,:
+    case _, (5, 6):
+        pass
+    case [[5], (6)], [7],:
+        pass
+    case _:
+        pass