fix bug in merge dicts: look for comma backwards

pyupgrade/_plugins/pep584.py

@@ -12,7 +12,6 @@
 from pyupgrade._data import State
 from pyupgrade._data import TokenFunc
 from pyupgrade._token_helpers import find_closing_bracket
-from pyupgrade._token_helpers import find_token
 
 
 def _replace_dict_brackets(i: int, tokens: List[Token]) -> None:
@@ -39,8 +38,10 @@ def _remove_double_star(i: int, tokens: List[Token]) -> None:
 
 
 def _replace_comma_with_pipe(i: int, tokens: List[Token]) -> None:
-    comma = find_token(tokens, i, ',')
-    tokens[comma] = Token('CODE', ' |')
+    j = i - 1
+    while not (tokens[j].name == 'OP' and tokens[j].src == ','):
+        j -= 1
+    tokens[j] = Token('CODE', ' |')
 
 
 @register(ast.Dict)
@@ -54,8 +55,7 @@ def visit_Dict(
 
     if all(key is None for key in node.keys) and len(node.values) > 1:
         yield ast_to_offset(node), _replace_dict_brackets
-        arg_count = len(node.values)
         for idx, arg in enumerate(node.values):
             yield ast_to_offset(arg), _remove_double_star
-            if idx < arg_count - 1:
+            if idx > 0:
                 yield ast_to_offset(arg), _replace_comma_with_pipe

tests/features/typing_584_test.py → tests/features/pep584_test.py

@@ -91,6 +91,19 @@ def test_fix_pep584_noop(s, version):
 
             id='Multiple lines, trailing comma',
         ),
+        pytest.param(
+            'x = {\n'
+            '    **{a: a for a  in range(3)},\n'
+            '    **b,\n'
+            '}\n',
+
+            'x = (\n'
+            '    {a: a for a  in range(3)} |\n'
+            '    b\n'
+            ')\n',
+
+            id='Dict comprehension within merge of dicts',
+        ),
     ),
 )
 def test_fix_pep584(s, expected):