allow trailing comma <3.6, test both postive/negative cases

parso/python/errors.py

@@ -800,9 +800,18 @@ class _ArglistRule(SyntaxRule):
 
             if argument.type == 'argument':
                 first = argument.children[0]
-                if argument.children[1].type in _COMP_FOR_TYPES and len(node.children) >= 2:
+                if (
-                    # a(a, b for b in c)
+                    argument.children[1].type in _COMP_FOR_TYPES
-                    return True
+                    and len(node.children) >= 2
+                ):
+                    if self._normalizer.version >= (3, 7):
+                        return True
+                    elif len(node.children) == 2 and node.children[1] == ",":
+                        # trailing comma allowed until 3.7
+                        pass
+                    else:
+                        return True
+
                 if first in ('*', '**'):
                     if first == '*':
                         if kw_unpacking_only:

test/test_python_errors.py

@@ -378,12 +378,17 @@ def test_repeated_kwarg():
 
 
 @pytest.mark.parametrize(
-    'source', [
+    ('source', 'no_errors', 'version'), [
-        'a(a for a in b,)'
+        ('a(a for a in b,)', True, '3.6'),
-        'a(a for a in b, a)',
+        ('a(a for a in b,)', False, '3.7'),
-        'a(a, a for a in b)',
+        ('a(a for a in b, a)', False, None),
-        'a(a, b, a for a in b, c, d)',
+        ('a(a, a for a in b)', False, None),
+        ('a(a, b, a for a in b, c, d)', False, None),
+        ('a(a for a in b)', True, None),
+        ('a((a for a in b), c)', True, None),
+        ('a(c, (a for a in b))', True, None),
+        ('a(a, (a for a in b), c)', True, None),
     ]
 )
-def test_unparenthesized_genexp(source):
+def test_unparenthesized_genexp(source, no_errors, version):
-    assert _get_error_list(source)
+    assert bool(_get_error_list(source, version=version)) ^ no_errors